xds: Fix import scripts deleting the wrong files, use of trap, and full git clone

The scripts used `git rev-parse --show-toplevel` so it appeared they
could be used from any directory. But references to "GIT_BASE_DIR"
weren't absolute, so it did matter the starting directory. And it
mattered in a big way for xds/import.sh as if you ran it from the
grpc-java directory it would delete the xds directory in grpc-java, not
third_party.

The trap that deleted the GIT_BASE_DIR was very broken. In addition to
potentially deleting the wrong directory, it was unnecessary because
that directory was in tmpdir. But you can only have one trap per signal,
so this unnecessary trap disabled the trap that deleted tmpdir.

The script needed a full clone because it needed to check out a specific
commit. To work with --depth 1 you have to use some convoluted syntax.
But just downloading a tar.gz is easy and seems should work fine on Mac.

protoc-gen-validate/import.sh didn't have the trap problem, but seemed
to have drifted from the other scritps. All the scripts were synced to
match.

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