chore: Remove orphaned z submodule reference from .gitmodules

3rd-party/z (rupa/z) was declared but the directory doesn't exist on
disk, isn't in the git index, and nothing sources it — zoxide replaced
it as the cd-frecency tool (see rc.d/zoxide). complete-alias and
fzf-git.sh are left as-is; they still need to be initialized via the
chezmoi-managed external config that clones this repo (separate task).

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "3rd-party/z"]
-	path = 3rd-party/z
-	url = https://github.com/rupa/z.git
 [submodule "3rd-party/complete-alias"]
 	path = 3rd-party/complete-alias
 	url = https://github.com/cykerway/complete-alias.git