Rename "crystal plate mail" to "crystal plate".

It makes no sense to make MAIL from crystal -- thin plates made of a
material that is on the brittle side would break in no time, and piercing
them to insert combining rings would chip it out quickly as the rings move
somewhat during use.

Making plate ARMOUR on the other hand is reasonable assuming you have proper
crystal.  (Yeah, yeah, "crystal" in the popular meaning, I know how most
solids look on the molecular level :p)

The tiles show plate armour rather than plate mail, too.

Conflicts:
	crawl-ref/source/dat/descript/items.txt