[Fix #604] Remove remove_reference checks from Rails/ReversibleMigration

[TonyArra]

Fixes #604

This removes the new check for remove_reference in Rails/ReversibleMigration (#592), as well as the recently added check for remove_belongs_to - an alias of remove_reference.

As discussed in #604, both of these statements are reversible, since their inverse methods are included in ActiveRecord::Migration::CommandRecorder

It is true that any options that were used in the add_reference/add_belongs_to statements need to also be included in the remove_reference statement; otherwise, a rollback will not properly restore the schema back to its previous state; however, the same could be said of several other reversible statements, such as remove_column.

For example, if I add a column with additional options, like default:

    add_column :comments, :position, :integer, default: 0

and then later remove that column without specifying those same options

    remove_column :comments, :position, :integer

then a rollback will not properly revert the schema to its previous state; it will be missing the default option.

Since there is no simple way to check that all of the options on an index, foreign key, constraint, or column are present in the remove_* statement, it seems impractical to have a cop for these methods. It will most likely result in more users disabling the cop entirely.

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Before submitting the PR make sure the following are checked:

• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[pirj]

Thanks!

[koic]

Can you add the changelog entry and squash your commits into one?

Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[TonyArra]

@koic done
Edit: appended implicit profile link to CHANGELOG.md

[koic]

Thanks!