With the release of GCC 6.5 the GCC 6 release branch has been closed
upstream and there won't be any further snapshots of this branch. (The
current/latest essentially is the same as the 6.5 release).

Hence remove the lang/gcc6-devel port which has been tracking snapshots
all along.

lang/gcc6 is a direct replacement and will be available for a longer
time. Note this is also the last version of GCC featuring the Java
front end.