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

Authored by martinvonz.

Description

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