Remove `BackwardCompatibility` module from `FilterManager` for rspec-3.0.
Update Changelog to indicate removal.
Remove scenario pertaining to overriding implicit filters.
Merge pull request #1222 from tundal45/1197_remove_backward_compatibility_module

Remove `BackwardCompatibility` module from `FilterManager` for rspec-3.0.
Rewrite changelog entry.

Changelog entries should be phrased in terms of how
the external behavior has changed rather than how
the implementation has changed.

[ci skip]
Add deprecation warning for `treat_symbols_as_metadata_keys_with_true_values`.

Add a custom message to notify users that this is now the default
behavior.
Clarify that the defult value is true and setting it as false has no effect.