Remove call to string-to-multibyte from nndoc.el

Remove call to string-to-multibyte from nndoc.el

* lisp/gnus/nndoc.el (nndoc-oe-dbx-type-p): My testing shows that
no matter whether we're in a unibyte or a multibyte buffer, doing
the looking-at here without the string-to-multibyte, we'll get a
match.  We did not get a match with the call in and if we were in
a unibyte buffer, but we presumably never are.