This completes what c774b28f (#9281)
started.  `integer-gmp-1.0` was added as an additional
`libraries/integer-gmp2` folder while retaining the ability to configure
GHC w/ the old `integer-gmp-0.5` to have a way back, and or the ability
to easily switch between old/new `integer-gmp` for benchmark/debugging
purposes.

This commit removes the old `libraries/integer-gmp` folder and moves
`libraries/integer-gmp2` into its place, while removing any mentions of
"gmp2" as well as the to support two different `integer-gmp` packages in
GHC's source-tree.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D769