gn build: Stop passing -DLLVM_LIBXML2_ENABLED to some targets

This is a remnant from before the gn build had a working config.h.

Defining LLVM_LIBXML2_ENABLED only for targets that depend on build/libs/xml is
nice in that only some of the codebase needs to be rebuilt when
llvm_enable_libxml2 changes -- but config.h already defines it and defining it
there and then redundantly a second time for some targets is worse than having
it just in config.h.

No behavior change.

Differential Revision: https://reviews.llvm.org/D56908

llvm-svn: 351758