The Logger severity predicates have existed since the [introduction of
Logger][1]. However, these methods only looked at the `level` instance
variable, so they did not work with the [thread safe implementation][2]
of temporary log levels in Rails.

Since then, the Logger severity predicates were [updated][3] to use the
`level` method instead of the instance variable, making Rails' severity
predicate overrides obsolete.

This commit removes Rails' custom severity predicates in favor of
Logger's implementation, since the new implementation was released in
Logger 1.4.2 and came bundled with Ruby 2.7.0.

[1]: ruby/logger@525b58d97eb7dd1375f59dec6e4f34ebff0c0ecb
[2]: rails/rails@629efb605728b31ad9644f6f0acaf3760b641a29
[3]: ruby/logger@7365c995bfde6cc52ad6c481e4c6fff32780025c