configure: Delete toolchain configuration logic from configure

With ghc-toolchain (!9263 (merged)) being merged, we will eventually want to configure toolchains exclusively with ghc-toolchain and drop all the logic for configuring toolchains in ./configure (this logic already lives is in the ghc-toolchain tool).

However, we won't drop the ./configure support right away. For some release cycle(s) we'll rather ship both options, and validate ghc-toolchain against ./configure.

This ticket tracks that eventual move to ghc-toolchain by default and deletion of configure logic.

There's also a preliminary MR (that might be a bit harsh to rebase in the future) in !10727

Should we milestone for 9.12?