fix(client): remove client operations for adding/removing callbacks

[tomlongridge]

Goal

Decision made that modifying delivery-time callbacks on the client doesn't seem very useful (as it's on the subsequent launch) and likely to cause confusion, so removing them from the API.

Changeset

• Removed from code and headers
• Removed tests for the methods specifically
• Updated expectations on e2e tests

[twometresteve]

Nit: The changelog entry includes the following. Technically true, but potentially confusing (suggest removing "Bugsnag and")

* Added `addOnSendBlock:`, `removeOnSendBlock:` and `clearOnSendBlocks` methods to `Bugsnag` 
  and `BugsnagConfiguration`.
  [#485](https://github.com/bugsnag/bugsnag-cocoa/pull/485)