cabal-testsuite: remove unused TestEnv.testRecordNormalizer field

The field `testRecordNormalizer :: String -> String` of the `TestEnv`
record seems to be a hook to arbitrarily reformat the recorded output
of a test.  However, it is ever only set to `id`.

On the other hand, being of function type, this field prevents us from
deriving a `Show` instance.  Thus, it is being removed as dead code.