gn build: Stop passing -DLLVM_LIBXML2_ENABLED to some targets

[thakis]

Summary

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.

[phosek]

LGTM

llvm/utils/gn/build/libs/xml/BUILD.gn

 import("//llvm/utils/gn/build/libs/xml/enable.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
 
 config("xml_config") {
   visibility = [ ":xml" ]
-  defines = [ "LLVM_LIBXML2_ENABLED" ]
   libs = [ "xml2" ]
   if (host_os == "mac") {
     include_dirs = [ "$mac_sdk_path/usr/include/libxml2" ]
   } else {
     include_dirs = [ "/usr/include/libxml2" ]
   }
 }
 
 group("xml") {
   if (llvm_enable_libxml2) {
     public_configs = [ ":xml_config" ]
   }
 }