fix: remove unused error-prone submodule setup

[proletarius101]

According to https://git-scm.com/docs/gitsubmodules, there must be a working directory at path/to/bar/ if there's a submodule.foo.path = path/to/bar entry in .gitmodules. Failure to have this will result in errors in certain build environments (e.g. Flatpak: standardnotes/app#516)

Same as in rust-windowing#1575

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality