Drop obsolete GHC versions 7.10.3, 8.0.2, and 8.4.3.

We keep the latest minor release of each one of the last 3 major releases,
which currently are GHC versions 8.2.2, 8.4.4, and 8.6.1. We also have
ghc-HEAD, but this doesn't count.

Dropping these compilers implied that we have to drop the corresponding
versions of ghcjs, too. We can also drop a shitload of obsolete compiler
patches that newer versions no longer need.

At some point, we can probably simplify the generic builder, too.