submodule API & "absorbgitdirs": remove "----recursive" option

Remove the "----recursive" option to "git submodule--helper
absorbgitdirs" (yes, with 4 dashes, not 2).

This option and all the "else" when "flags &
ABSORB_GITDIR_RECURSE_SUBMODULES" is false has never been used since
it was added in f6f8586140 (submodule: add absorb-git-dir function,
2016-12-12), which we'd have had to do as "----recursive", a
"--recursive" would have errored out.

It would be nice to follow-up with an optbug() assertion to
parse-options.c for such funnily named options, I manually validated
that this was the only long option whose name started with "-", but
let's skip adding such an assertion for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

submodule.h

@@ -164,9 +164,7 @@ void submodule_unset_core_worktree(const struct submodule *sub);
  */
 void prepare_submodule_repo_env(struct strvec *env);
 
-#define ABSORB_GITDIR_RECURSE_SUBMODULES (1<<0)
-void absorb_git_dir_into_superproject(const char *path,
-				      unsigned flags);
+void absorb_git_dir_into_superproject(const char *path);
 
 /*
  * Return the absolute path of the working tree of the superproject, which this