parse-remote: remove this now-unused library

The previous two commits removed the last use of a function in this
library, but most of it had been dead code for a while[1][2]. Only the
"get_default_remote" function was still being used.

Even though we had a manual page for this library it was never
intended (or I expect, actually) used outside of git.git. Let's just
remove it, if anyone still cares about a function here they can pull
them into their own project[3].

1. Last use of error_on_missing_default_upstream():
   d03ebd411c ("rebase: remove the rebase.useBuiltin setting",
   2019-03-18)

2. Last use of get_remote_merge_branch(): 49eb8d39c7 ("Remove
   contrib/examples/*", 2018-03-25)

3. https://lore.kernel.org/git/87a6vmhdka.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -20,7 +20,6 @@ USAGE="[--quiet] [--cached]
 OPTIONS_SPEC=
 SUBDIRECTORY_OK=Yes
 . git-sh-setup
-. git-parse-remote
 require_work_tree
 wt_prefix=$(git rev-parse --show-prefix)
 cd_to_toplevel