Remove prefer_equal_for_default_values from the recommended.yaml set

[parlough]

Now that the package requires SDK ^3.0.0, this lint does nothing since it's not reported by the Dart 3 SDK, which instead reports the deprecated_colon_for_default_value diagnostic for 2.12-2.19 language versions.

\cc @pq @devoncarew

[lrhn]

If we have made the SDK requirement for this package ^3.0.0, then the lint is definitely not needed.

If the same version of the lints package can be used by older SDKs, then it might be worth retaining, as long as we're sure that the lint doesn't cause double-reports in SDKs where the analyzer warns itself. (I guess that when the analyzer knows that it reports useing :, it can also know to ignore this lint.)

[devoncarew]

Removal approved.