[bazel] Remove unused dependency on libxml2

The Bazel configs don't set `LLVM_ENABLE_LIBXML2`, so this was never usable to
begin with.

On systems without static `libxml2.a` this made `lld` runtime-dependent on an
unused, non-hermetic `libxml2.so`.

Reviewed By: MaskRay, GMNGeoffrey

Differential Revision: https://reviews.llvm.org/D143351