Tweak the mass draining card effect to more closely match

monsters::drain_exp(). They should eventually be consolidated, as soon as I figure out how to scale the power.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8971 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on February 7, 2009
LXU3I6BZOJ32TJQPVWPJ536AW3CZEXJGNJIS5TE572PJHYFC76GAC
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