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: lme (mentor)
Differential Revision: https://reviews.freebsd.org/D10459