Remove deprecated methods and rake tasks

[Vitalina-Vakulchyk]

Remote methods and rake tasks, that marked as deprecated.

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[rabotyaga]

Suggested change

	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `Chewy::Index.index_params`, use `Chewy::Index.specification_hash` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `Chewy::Index.derivable_index_name`, use `Chewy::Index.derivable_name` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `Chewy::Index.build_index_name`, use `Chewy::Index.index_name` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `Chewy::RakeHelper.reset_index`, use `Chewy::RakeHelper.reset` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `Chewy::RakeHelper.reset_all`, use `Chewy::RakeHelper.reset` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `Chewy::RakeHelper.update_index`, use `Chewy::RakeHelper.update` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `Chewy::RakeHelper.update_all`, use `Chewy::RakeHelper.update` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `rake chewy:apply_changes_from`, use `rake chewy:journal:apply` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Remove `rake chewy:clean_journal`, use `rake chewy:journal:clean` instead ([@Vitalina-Vakulchyk][])
	* [#769](https://github.com/toptal/chewy/pull/769): **(Breaking)** Removal of deprecated methods and rake tasks ([@Vitalina-Vakulchyk][]):
	* `Chewy::Index.index_params` is removed, use `Chewy::Index.specification_hash` instead
	* `Chewy::Index.derivable_index_name` is removed, use `Chewy::Index.derivable_name` instead
	* `Chewy::Index.default_prefix` is removed, use `Chewy::Index.prefix` instead
	* `Chewy::Index.build_index_name` is removed, use `Chewy::Index.index_name` instead
	* `Chewy::RakeHelper.reset_index` is removed, use `Chewy::RakeHelper.reset` instead
	* `Chewy::RakeHelper.reset_all` is removed, use `Chewy::RakeHelper.reset` instead
	* `Chewy::RakeHelper.update_index` is removed, use `Chewy::RakeHelper.update` instead
	* `Chewy::RakeHelper.update_all` is removed, use `Chewy::RakeHelper.update` instead
	* `rake chewy:apply_changes_from` is removed, use `rake chewy:journal:apply` instead
	* `rake chewy:clean_journal` is removed, use `rake chewy:journal:clean` instead

[bbatsov]

Probably something like this should be added to the upgrade/migration notes as well.

[rabotyaga]

Probably something like this should be added to the upgrade/migration notes as well.

Actually, they all have been deprecated since 2017. I think it should be safe just to drop them finally, with a note in the CHANGELOG only.

[Vitalina-Vakulchyk]

Currently, I have just updated CHANLOG again.
I can do more changes if needed.