gcc: stop adding -fno-common for checking builds

Originally added in r0-44646-g204250d2fcd084 and r0-44627-gfd350d241fecf6 whic
moved -fno-common from all builds to just checking builds.

Since r10-4867-g6271dd984d7f92, GCC defaults to -fno-common. There's no need
to pass it specially for checking builds.

We could keep it for older bootstrap compilers with checking but I don't see
much value in that, it was already just a bonus before.

gcc/ChangeLog:
	* Makefile.in (NOCOMMON_FLAG): Delete.
	(GCC_WARN_CFLAGS): Drop NOCOMMON_FLAG.
	(GCC_WARN_CXXFLAGS): Drop NOCOMMON_FLAG.
	* configure.ac: Ditto.
	* configure: Regenerate.

gcc/d/ChangeLog:
	* Make-lang.in (WARN_DFLAGS): Drop NOCOMMON_FLAG.