Drop 9.x hack for USES=compiler:c++14-lang
On x86 architectures when base compiler doesn't support C++14
USES=compiler prefers Clang. As only lang/gcc* provide C++14 library
outside of base some ports need to define USE_GCC. However, adding it
would require ugly bsd.port.options.mk conditionals thus FAVORITE_COMPILER
was used. As /stable/9 reached EOL we no longer need to support ancient
libstdc++ on x86.