usage.c: drop set_error_handle()

usage.c: drop set_error_handle()

The set_error_handle() function was introduced by 3b331e926
(vreportf: report to arbitrary filehandles, 2015-08-11) so
that run-command could send post-fork, pre-exec errors to
the parent's original stderr.

That use went away in 79319b194 (run-command: eliminate
calls to error handling functions in child, 2017-04-19),
which pushes all of the error reporting to the parent.
This leaves no callers of set_error_handle(). As we're not
likely to add any new ones, let's drop it.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>