Bug 1918691 - Remove metadata file for two WPT tests that will pass in beta r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D222359

testing/web-platform/meta/css/filter-effects/backdrop-filter-reference-filter-mutated.html.ini

@@ -1,4 +0,0 @@
-[backdrop-filter-reference-filter-mutated.html]
-  bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1916097
-  expected:
-    if not nightly_build: FAIL

testing/web-platform/meta/css/filter-effects/effect-reference-on-transparent-element.html.ini

@@ -1,4 +0,0 @@
-[effect-reference-on-transparent-element.html]
-  bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1906212
-  expected:
-    if not nightly_build: FAIL