[headless] Remove dependency on //content/test:browsertest_support

This dependency is not needed and it causes link step problems,
see here: https://ci.chromium.org/ui/p/chromium/builders/try/linux-rel/1692601/overview

Symbol main() is defined in both headless_test_launcher.cc and
content_test_launcher.cc which is included in //content/test:browsertest_support target

Change-Id: I26bde8335ef12a2219ae133dd604510c261ffd82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5273277
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Peter Kvitek <kvitekp@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1257008}

headless/BUILD.gn

@@ -758,7 +758,6 @@ test("headless_browsertests") {
     "//components/policy/core/browser",
     "//components/security_state/content",
     "//components/version_info",
-    "//content/test:browsertest_support",
     "//content/test:test_support",
     "//gin",
     "//net:test_support",