Revert "fix: support overwrite when merge config. (#1394)"

[Jint-lzxy]

This reverts commit 7056741.

Sorry for the noise but I'm a lil confused about this new option...? Like idk if the performance difference between these two is even that big, and maybe it's not actually working as expected?

[ayamir]

overwrite option is the only solution to delete pre-defined something, like I want to define edgy.nvim left region with NeoTree rather than NvimTree. Otherwise I need to use function to repeat lots of content the same with defaults.

[Jint-lzxy]

overwrite option is the only solution to delete pre-defined something

But I think this might break some users' expectations of the loader right 🤔 Also think I mentioned vim.NIL elsewhere too, it's like a user object so maybe we can use that to signal "erasure" of the field instead?

[ayamir]

Marking a option as nil is not enough, overwrite is an irreplaceable way to acheive delete and re-define.