toolchain.eclass: [QA] Remove meaningless USE=multislot, #584610

toolchain.eclass: [QA] Remove meaningless USE=multislot, #584610

The USE=multislot as defined partially by toolchain.eclass and partially
by sys-devel/gcc was used for two purposes:

- enabling build-time (only) blockers on old gcc versions -- which do
  not make any sense because they are build-time only and there is no
  technical reason for two gcc version ranges not to be installed at
  the same time, both at build time and at run time,

- changing behavior of post-install wrt conditional gcc-config calls.

Both cases are invalid use of USE flags, considering that the flag does
not affect the installed files in any way.