Drop unused exception local

Summary: It's unused, remove it.

Reviewed By: yfeldblum

Differential Revision: D7462683

fbshipit-source-id: 604c5691d69f2576c83c25350d29515efbc77a7b

folly/experimental/logging/LogConfigParser.cpp

@@ -72,7 +72,7 @@ bool parseJsonLevel(
     try {
       result = stringToLogLevel(levelString);
       return true;
-    } catch (const std::exception& ex) {
+    } catch (const std::exception&) {
       throw LogConfigParseError{to<string>(
           "invalid log level \"",
           levelString,