quote: drop sq_dequote_to_argv()

quote: drop sq_dequote_to_argv()

The last caller went away in f9dbb64fad (config: parse more robust
format in GIT_CONFIG_PARAMETERS, 2021-01-12), when we switched to using
sq_dequote_step().

The "to_argv()" form is not a great interface. If you care about raw
speed, then sq_dequote_step() lets you work incrementally without extra
allocations. If you care about simplicity, then sq_dequote_to_strvec()
puts the result in an encapsulated data structure. With sq_dequote_to_argv(),
you have a data dependency on the original string but still have to
remember to manually free the argv array itself (but not its elements).

So it's sort of a worst-of-both-worlds middle ground. Let's get rid of
it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>