sparse-index.c: remove set_index_sparse_config()

sparse-index.c: remove set_index_sparse_config()

Remove the set_index_sparse_config() function by folding it into
set_sparse_index_config(), which was its only user.

Since 122ba1f7b52 (sparse-checkout: toggle sparse index from builtin,
2021-03-30) the flow of this code hasn't made much sense, we'd get
"enabled" in set_sparse_index_config(), proceed to call
set_index_sparse_config() with it.

There we'd call prepare_repo_settings() and set
"repo->settings.sparse_index = 1", only to needlessly call
prepare_repo_settings() again in set_sparse_index_config() (where it
would early abort), and finally setting "repo->settings.sparse_index =
enabled".

Instead we can just call prepare_repo_settings() once, and set the
variable to "enabled" in the first place.

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