[COEP] Fix malformed .headers in WPT

The HTTP header is malformed due to the trailing semicolon. Remove it.

Bug: None
Change-Id: Ifb6042fa169bdf373be418fab89c67ad94e105fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151777Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759928}

third_party/blink/web_tests/external/wpt/html/cross-origin-embedder-policy/resources/reporting-empty-frame.html.headers

 cross-origin-embedder-policy: require-corp; report-to="endpoint"
-cross-origin-embedder-policy-report-only: require-corp; report-to="report-only-endpoint";
+cross-origin-embedder-policy-report-only: require-corp; report-to="report-only-endpoint"