[gn build] remove NOSORT from clang/Headers/BUILD.gn

Having the sync script work for this file seems better
than matching the order of headers in the cmake file.
Also, not having to manually sort the list is nice, even
if gn's automated sorting doesn't quite match the artisanal
order in the cmake file.

llvm/utils/gn/secondary/clang/lib/Headers/BUILD.gn

@@ -55,9 +55,6 @@ copy("arm_headers") {
 copy("Headers") {
   deps = [ ":arm_headers" ]
 
-  # NOSORT
-  # Tell `gn format` to not reorder the sources list: Its order matches CMake,
-  # and the ordering is alphabetical but ignores leading underscores.
   sources = [
     "arm_cmse.h",
     "adxintrin.h",