Remove reference to gyp-mac-tool in .gitignore.

This file is no longer used, and even when it was (at least recently) wasn't placed in a git-controlled directory.

Review-Url: https://codereview.chromium.org/2287693002
Cr-Commit-Position: refs/heads/master@{#414817}

.gitignore

@@ -183,7 +183,6 @@ vs-chromium-project.txt
 /google_apis/internal
 /googleurl
 /gpu/gles2_conform_test
-/gyp-mac-tool
 /infra/.recipe_deps
 /internal_gyp
 /ios/third_party/earl_grey/src