Remove some more dead IN_GCC code.

Have fixed mars.h in #2196 and made changes on gdc's side so that d_float80 is safe to use in the frontend.  No longer need to explicitly use real_t.

The change to ```lexer.h``` hasn't been fixed in gdc.  But I am looking to remove all constructors defined in gdc's longdouble class to allow it to be possible.