Remove USE_GCC_RUNTIME?

[nia <nia%NetBSD.org@localhost>]

Hey, since the recent talks where people expressed a distate for
GCC-specific hacks in pkgsrc, I wonder whether it's worth bringing
this variable up:

# USE_GCC_RUNTIME
#       Packages which build shared libraries but do not use libtool to
#       do so should define this variable.  It is used to determine whether
#       the gcc runtime should be depended upon when a user has enabled
#       USE_PKGSRC_GCC_RUNTIME.

In my opinion this is a historical wart - not using libtool (while
unfortunate) is increasingly common with more packages using cmake/meson
instead. In my eyes the gcc runtime should be depended upon whenever
USE_PKGSRC_GCC_RUNTIME is set.