parse-remote: remove this now-unused library

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>