Bug 1960025: Remove no longer needed cxxflags override. r=platform-i18n-reviewers,nordzilla

Updating to a more recent ICU4X seems to have fixed this issue, so we no longer
need to add this flag.

Differential Revision: https://phabricator.services.mozilla.com/D245273

intl/lwbrk/moz.build

@@ -48,10 +48,4 @@ LOCAL_INCLUDES += [
     "/intl/icu_capi/bindings/cpp",
 ]
 
-# Disable warnings when including C++ headers of ICU4X.
-# - https://github.com/rust-diplomat/diplomat/issues/277
-CXXFLAGS += [
-    "-Wno-mismatched-tags",
-]
-
 FINAL_LIBRARY = "xul"