Put monster speed calculation back in _define_zombie(), and fix problem

where the speed was being downgraded for zombies twice.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9359 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on March 6, 2009
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