lang/ghc: Continue with cleanups
- Remove INSTALL_TARGET as it is now the same for all GHCs and matches the default
- Remove GHCs 8.10.7 from distinfo, these are unused now
- Remove USE_PERL5
- Stop generating build.mk, it isn't used by Hadrian build system
- Move BOOT_SCRIPT definition into the slave port
- Dedup BOOT_DIR