Remove include/gpu/mtl from include directories.

Prefer to use "mtl/" to set apart such includes and lessen the include
directory pollution. In addition no users of Skia want to add this
include directory to their includes list, so this change keeps that
setup working by having Skia build this way as well.

Change-Id: I5742e2c981e4cdb95b5af18dd39ec877162788de
Reviewed-on: https://skia-review.googlesource.com/129655
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

BUILD.gn

@@ -107,9 +107,6 @@ skia_public_includes = [
 if (skia_enable_atlas_text) {
   skia_public_includes += [ "include/atlastext" ]
 }
-if (skia_use_metal) {
-  skia_public_includes += [ "include/gpu/mtl" ]
-}
 
 # Skia public API, generally provided by :skia.
 config("skia_public") {