filemerge: remove leftover documentation of removed argument

I removed the `premerge` argument in
https://phab.mercurial-scm.org/D11859.

Differential Revision: https://phab.mercurial-scm.org/D11925

mercurial/filemerge.py

@@ -1028,7 +1028,6 b' def _maketempfiles(repo, fco, fca, local'
 def filemerge(repo, wctx, mynode, orig, fcd, fco, fca, labels=None):
     """perform a 3-way merge in the working directory
 
-    premerge = whether this is a premerge
     mynode = parent node before merge
     orig = original local filename before merge
     fco = other file context