Remove gcc run dependency

- ghc uses USE_GCC=yes and when USE_GCC=yes implies a different one of
  the lang/gcc* ports (which will happen after PR 218330 or when the
  user sets a different version of GCC via Mk/bsd.default-versions.mk),
  devel/stacks pulls in two conflicting ports - lang/gcc and lang/gcc5 -
  in the first case, or pulls in the wrong port for its own dependency.

- Some Haskell libraries/programs require gcc in the PATH, but not all
  do; the port should not have a gcc dependency for this reason alone.

PR:		218482
Reported by:	gerald
Approved by:	??? (mentor)
Differential Revision:	https://reviews.freebsd.org/D?????