Update in-tree GMP to 6.3.0

This MR updates in-tree GMP to 6.3.0 (see gmp-tarballs!6 (merged)) and removes some obsolete logic in the tree. See release notes of GMP for the changes, 6.3.0 includes important bugfixes that previously we had to manually cherry pick in our own tarball.

Since this involves bumping a blob (dangerous!), to avoid me being another Jia Tan: please do take a look at the gmp-tarballs MR and run the build-nodoc-tarball script yourself and compare the extracted gmp-6.3.0-nodoc.tar file with my MR, it's supposed to be bit-by-bit reproducible.