Remove `-Xclang-only=` as that's not supported in the Debian LLVM
toolchain. Since clang is specified in the ../rules file, we don't need
to worry about the compatibility with other compilers.

--- a/libchrome/BUILD.gn
+++ b/libchrome/BUILD.gn
@@ -82,7 +82,7 @@ config("libchrome_config") {
     "-Wno-narrowing",
     "-Wno-unreachable-code-return",
     "-Wno-unused-local-typedefs",
-    "-Xclang-only=-Wno-char-subscripts",
+    "-Wno-char-subscripts",
   ]
 
   # Address sanitizer + coverage builds do not support -z,defs.