Bug 1730759: Remove unreachable return statement from KeyEventHandler::KeyToMask. r=masayuki

This return statement is just a copy of the 'default' case in the preceding
switch statement.  It's redundant and unreachable because every case in the
switch statement has a return statement.

clang correctly warns that it's unreachable, so this patch fixes a clang build
warning.

Differential Revision: https://phabricator.services.mozilla.com/D125595

dom/events/KeyEventHandler.cpp

@@ -526,7 +526,6 @@ int32_t KeyEventHandler::KeyToMask(int32_t key) {
     default:
       return cControl | cControlMask;
   }
-  return cControl | cControlMask;  // for warning avoidance
 }
 
 // static