[wayland][libxml2] Remove libxml2 dependency

We'd like to remove the libxml2 dependency as it's hard to maintain, and
wayland only uses it for this binary tool that we do not use. Remove the
source sets that require libxml2.

Bug: 431804801
Change-Id: Ie5ee2583e2e6f38a7a5f198fe9c06a4ad5140ce9
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/wayland/+/1321667
Reviewed-by: Craig Stout <cstout@google.com>