Yes, you can remove it. It is a spoke protector, often derisively referred to here as a dork disk or, in the old days, pie plate.

Just make sure your derailleur's limit screw is set to prevent the derailleur cage from overshooting the large inboard cog and you're good to go. If the bike ever falls on the derailleur side or the derailleur or its hanger suffers some other trauma, be sure to check that you still cannot shift it into the spokes before you resume riding it.