Bug 1605537 - remove coverage and warning flags from wasm flags; r=firefox-build-system-reviewers,rstewart

Warning flags might have been generated for a different compiler, so we
can't use them here.  The same argument applies to coverage flags.

Depends on D58070

Differential Revision: https://phabricator.services.mozilla.com/D58071

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/frontend/context.py

@@ -591,13 +591,8 @@ class WasmFlags(TargetCompileFlags):
              ('WASM_CFLAGS', 'WASM_CXXFLAGS', 'WASM_LDFLAGS')),
             ('WARNINGS_AS_ERRORS', self._warnings_as_errors(),
              ('WASM_CXXFLAGS', 'WASM_CFLAGS', 'WASM_LDFLAGS')),
-            ('WARNINGS_CFLAGS',
-             context.config.substs.get('WARNINGS_CFLAGS'),
-             ('WASM_CFLAGS', 'WASM_LDFLAGS')),
             ('MOZBUILD_CFLAGS', None, ('WASM_CFLAGS',)),
             ('MOZBUILD_CXXFLAGS', None, ('WASM_CXXFLAGS',)),
-            ('COVERAGE', context.config.substs.get('COVERAGE_CFLAGS'),
-             ('WASM_CXXFLAGS', 'WASM_CFLAGS')),
             ('WASM_CFLAGS', context.config.substs.get('WASM_CFLAGS'),
              ('WASM_CFLAGS',)),
             ('WASM_CXXFLAGS', context.config.substs.get('WASM_CXXFLAGS'),