input: don't print warning if certain internal keys are not bound

This affects MOUSE_MOVE and MOUSE_LEAVE. Both are needed internally
(such as for the OSC), but not really useful for input.conf. Since the
warning has the purpose of notifying the user that a key is unmapped and
what key name to use for setting up a binding in input.conf, the warning
is rather useless in this case. It's also annoying in combination with
the
--no-input-default-bindings option, since that removes the default
bindings to "ignore" for these keys.