dir: remove struct path_simplify

dir: remove struct path_simplify

Teach simplify_away() and exclude_matches_pathspec() to handle struct
pathspec directly, eliminating the need for the struct path_simplify.

Also renamed the len parameter to pathlen in exclude_matches_pathspec()
to match the parameter names used in simplify_away().

Signed-off-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>