submodule: remove unnecessary unabsorbed fallback

submodule: remove unnecessary unabsorbed fallback

In get_submodule_repo_for(), there is a fallback code path for the case
in which a submodule has an unabsorbed gitdir. (See the documentation
for "git submodule absorbgitdirs" for more information about absorbed
and unabsorbed gitdirs.) However, this code path is unnecessary, because
such submodules are already handled: when the fetch_task is created in
fetch_task_create(), it will create its own struct submodule with a path
and name, and repo_submodule_init() can handle such a struct.

This fallback was introduced in 26f80ccfc1 ("submodule: migrate
get_next_submodule to use repository structs", 2018-12-05). It was
unnecessary even then, but perhaps it escaped notice because its parent
commit d5498e0871 ("repository: repo_submodule_init to take a submodule
struct", 2018-12-05) was the one that taught repo_submodule_init() to
handle such created structs. Before, it took a path and always checked
.gitmodules, so it truly would have failed if there were no entry in
.gitmodules.

(Note to reviewers: in 26f80ccfc1, the "own struct submodule" I
mentioned is in get_next_submodule(), not fetch_task_create().)

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>