scmutil: remove special handling of pats==("",) in matchandpats() (API)

I don't know under what circumstances we used to pass that value, but
we don't seem to do it now.

Differential Revision: https://phab.mercurial-scm.org/D5921

mercurial/scmutil.py

@@ -795,8 +795,6 b' def matchandpats(ctx, pats=(), opts=None'
     '''Return a matcher and the patterns that were used.
     The matcher will warn about bad matches, unless an alternate badfn callback
     is provided.'''
-    if pats == ("",):
-        pats = []
     if opts is None:
         opts = {}
     if not globbed and default == 'relpath':