Log Message:
remove gcc48/gcc49.

use of these packages was disabled in the pkgsrc infrastructure in
january because they were causing problems on platforms with older
compilers that can build gcc6 just fine:
glibc + FORTIFY + gcc48,gcc49,gcc5 = build failures.

gcc48 and newer require a c++98 compiler, same as all gcc versions up
to 11, so are not useful for bootstrapping.

gcc5 has additional Ada bits, someone needs to determine if they're
useful before it can go.