Revert "Not scanning for alerts when message processing is disabled."

This reverts commit e3112d6a49f73d9d9b79e31f085e003de06af006.

This has been reverted because it is semantically wrong to disable
checking for alerts when message processing is disabled, because even
if it is disabled on master (where the alert scanning is run) it might
be enabled on other nodes, so scanning is still necessary.

Conflicts:
	graylog2-server/src/main/java/org/graylog2/periodical/AlertScannerThread.java