deskutils/growl-for-linux: Drop special casing of GCC 4.2

Remove code specific to GCC 4.2 (as system compiler) which is not
necessary any longer: USES=compiler never pulls that in any more.

This reverts most of commits 401f96f227521c4527ea9c3887b10fb4bd7ff24b
and 92e22122e819280ea0c18de3ddd7a8b8a25a3ead from 2018/2019.

Approved by: nivit (maintainer)