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)

• Tested on all platforms changed
• 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

[MarijnS95]

Is this submodule even used? It points to a repository that has been archived for quite some time and is superseded by multiple tools.

[MarijnS95]

Ah, @kchibisov also assigned me as reviewer, likely to check. No, I am not aware of any use of apk-builder, the CI uses cargo-apk which it installs ad-hoc (and the readme documents this for users. The submodule should be removed instead.

[proletarius101]

Changed to remove that submodule
@MarijnS95

[kchibisov]

Ah, @kchibisov also assigned me as reviewer, likely to check. No, I am not aware of any use of apk-builder

Thanks, for confirming, that it's redundant. I also had a feeling that this file was just laying around without anyone noticing it.

@proletarius101 thanks for removing it :p