feat(sys_mon): remove `current_function`

Since we now output `current_stacktrace`, `current_function` is now
redundant.

apps/emqx/src/emqx_vm.erl

@@ -61,7 +61,6 @@
                          ]).
 
 -define(PROCESS_INFO_KEYS, [initial_call,
-                            current_function,
                             current_stacktrace,
                             registered_name,
                             status,