tests: submodule: fix tests for cloning submodules

The test submodule::add::homemade_clone unfortunately doesn't test
what's expected, but does instead clone the submodule to a directory
that is outside of the parent repository. Fixing this by cloning to the
correct location isn't possible, though, as `git_submodule_add_setup`
will have pre-created a ".git" file already, which will cause
`git_clone` to error out.

As it's not possible to perform the clone without fiddling around with
the repo's layout, let's just remove this test as that is in fact what
the new `git_submodule_clone` function is for.

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