Remove reference to GYP_DEFINES

This was removed in https://crrev.com/c/1361804. I missed this reference
though.

Bug: 718157
Change-Id: Idbddfec89d5c143c494df1f5572cb82f631baf62
Reviewed-on: https://chromium-review.googlesource.com/c/1371192Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615691}

third_party/blink/tools/blinkpy/web_tests/port/base.py

@@ -1128,7 +1128,6 @@ class Port(object):
         if self.host.platform.is_win():
             variables_to_copy += [
                 'PATH',
-                'GYP_DEFINES',  # Required to locate win sdk.
             ]
 
         for variable in variables_to_copy: