templater: stop accepting a single style to stylemap()

The code seems to have been for compatibility across d3dbdca92458
(hgweb: don't choke when an inexistent style is requested (issue1901),
2009-11-12).

Differential Revision: https://phab.mercurial-scm.org/D8844

mercurial/templater.py

@@ -1089,9 +1089,6 b' def stylemap(styles, path=None):'
     if path is None:
         path = templatedir()
 
-    if isinstance(styles, bytes):
-        styles = [styles]
-
     if path is not None:
         for style in styles:
             # only plain name is allowed to honor template paths