[R-package] Remove lgb.prepare() and lgb.prepare_rules()

[jameslamb]

The R package currently has 4 functions for converting non-numeric inputs to numeric values for use with LightGBM training:

• lgb.prepare() (docs)

• lgb.prepare2() (docs)

• lgb.prepare_rules() (docs)

• lgb.prepare_rules2() (docs)

The *2() versions of these functions accomplish everything that their predecessors do, and in a better way. I think that lgb.prepare() and lgb.prepare_rules() should be deprecated in the next 2.x release, and that they should be removed completely in 3.0.0.

This will simplify the interface of the R package for users and reduce the maintenance burden for maintainers.

@Laurae2 @guolinke what do you think?

[jameslamb]

Just put this in #2302 . Will close it once I hear from the maintainers tagged in the description about whether this is a good idea.

[Laurae2]

@jameslamb as LightGBM does not crash anymore when using an R integer matrix, we can deprecate the non *2() versions.

[jameslamb]

@Laurae2 ok thanks! I'll open a PR with a deprecation warning, and then when we do 3.0.0 (#3071 ) we can remove them

[jameslamb]

Closing for now since we've agreed on direction. This issue is in #2302 with other features.

[github-actions]

This issue has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.