src/xdisp.c: Remove some unused parameters.

src/xdisp.c: Remove some unused parameters.

* xdisp.c (display_count_lines): Remove parameter `start',
  unused since 1998-01-01T02:27:27Z!rms@gnu.org.  All callers changed.
  (get_char_face_and_encoding): Remove parameter `multibyte_p',
  unused since 2008-05-14T01:40:23Z!handa@m17n.org.  All callers changed.
  (fill_stretch_glyph_string): Remove parameters `row' and `area',
  unused at least since Kim's GUI unification at 2003-03-16T20:45:46Z!storm@cua.dk
  and thereabouts.  All callers changed.
  (get_per_char_metric): Remove parameter `f', unused since
  2008-05-14T01:40:23Z!handa@m17n.org.  All callers changed.