src/xdisp.c: Remove some unused parameters.

src/xdisp.c: Remove some unused parameters.

* xdisp.c (display_count_lines): Remove parameter `start',
  unused since revno:20537 (1998-01-01).  All callers changed.
  (get_char_face_and_encoding): Remove parameter `multibyte_p',
  unused since revno:87605 (2008-05-14).  All callers changed.
  (fill_stretch_glyph_string): Remove parameters `row' and `area',
  unused at least since Kim's GUI unification at revno:50135 (2003-03-16)
  and thereabouts.  All callers changed.
  (get_per_char_metric): Remove parameter `f', unused since
  revno:87605 (2008-05-14).  All callers changed.