Remove unused FTE related parts on the macOS Gitian descriptors
We disabled FTE packaging in #13252 (moved) due to code-signing issues. No progress has made on this issue (tracked in #18495 (closed)) since. We should back out the FTE part and all the related Python parts and building GMP as both are only needed for FTE. This would simplify our descriptors considerably and would avoid building stuff we don't need and would make it easier to migrate to a rbm
based build system. Once #18495 (closed) got resolved we can include FTE for macOS again.