Bug 1672946 - Revert disabling optimizations on mfbt tests that accidentally landed. r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D94576

mfbt/tests/moz.build

@@ -109,5 +109,3 @@ USE_LIBS += [
 
 if CONFIG["CC_TYPE"] in ("clang", "gcc"):
     CXXFLAGS += ["-Wno-error=shadow"]
-
-CXXFLAGS += ["-O0"]