pathspec: remove PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE flag

pathspec: remove PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE flag

Since (ae8d08242 pathspec: pass directory indicator to
match_pathspec_item()) the path matching logic has been able to cope
with submodules without needing to strip off a trailing slash if a path
refers to a submodule.

Since the stripping the trailing slash is no longer necessary, remove
the PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE flag.  In addition, factor
out the logic which dies if a path decends into a submodule so that it
can still be used as a check after a pathspec struct has been
initialized.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>