main/vim: remove filetype override for APKBUILD

Vim's upstream defaults already set filetype=sh for APKBUILD files, so this is redundant and effectively a no-op.

Context

Vim plugins such as ALE or nvim-lspconfig currently enable bash-specific linters when filetype=bash. On the other hand, ALE only enables APKBUILD-specific checks if filetype=apkbuild.

I am working on patches for upstream that introduce a dedicated filetype, apkbuild. This filetype will use the same sh highlight rules (so highlighting remains unchanged).

This override here negates the effect of such patches. I'd rather remove them now and rely entirely on what upstream does.