Remove unnecessary dependency for wpt builders

Platform folder only has baselines in it. In future we will put
platform specific metadata to a different folder.

Bug: 1299834
Change-Id: I3b13fa4e9576f48f9b3a41cc871bc4dea680b41d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3582725
Reviewed-by: Rakib Hasan <rmhasan@google.com>
Commit-Queue: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#992337}

third_party/blink/tools/BUILD.gn

@@ -23,9 +23,6 @@ group("wpt_tests_isolate") {
     # the test runner codebase, manifest file, and the tests themselves.
     "//third_party/blink/web_tests/external/",
 
-    # The web_tests/platform directory contains platform specific WPT baselines.
-    "//third_party/blink/web_tests/platform/",
-
     # Include a single file from wpt_internal only to initialize that directory.
     # We don't need to run these tests but still need to create a MANIFEST here.
     # See crbug.com/1018829 for more details.