Remove <em> from autoblocker message

It doesn't work (the limited parser used for edit/block summaries doesn't
understand <em>) and it is not necessary (things shouldn't be both in
quotes and italics, one of these choices is enough).

Follow-up to If84e78e413fd042ef9451c68df2321fd52a35c79.

Change-Id: I505a486624f1891fb8b708862692637d4ec42db8
Merge "Remove <em> from autoblocker message"