Remove unneeded file.

public_headers.gypi was never needed by Skia proper, and was almost
always out of date. Its actual user no longer needs it, so stop
updating it with every other change to public headers.

BUG=skia:2350

Review URL: https://codereview.chromium.org/683123003

gyp/public_headers.gypi

@@ -1,10 +0,0 @@
-# DO NOT MODIFY. This file is deprecated. See skbug.com/2350
-#
-{
-  'variables': {
-    'header_filenames': [
-        # Do not modify. This is only here until we remove its reference
-        # in chromium.
-    ],
-  },
-}