[Mac] Stop including generate_diffs in the installer packaging tools.

The script is no longer used by the signing pipeline and will be
removed in https://chrome-internal-review.googlesource.com/c/654803.

Bug: 660598
Change-Id: Ia80e4dfbf186c2142b8aff021e3842f87c44a2ad
Reviewed-on: https://chromium-review.googlesource.com/1147472Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577644}

chrome/installer/mac/BUILD.gn

@@ -109,7 +109,6 @@ copy("copies") {
       "internal/chrome_dmg_background.png",
       "internal/chrome_dmg_dsstore",
       "internal/chrome_dmg_icon.icns",
-      "internal/generate_diffs",
       "internal/generate_dmgs",
       "internal/signing_common.sh",
     ]