Ignore completeItemsIsIncomplete and always filter items in omnifunc

[bennyyip]

According to this, rust-analyzer unconditionally set IsIncomplete to true for faster autocomplete response.
This results in omnicomplete always show a unfiltered list, which contains a lot of useless entries.
It should be OK always filter the list in omnifunc, because speed doesn't matter as much as in autocomplete. I can't see the necessity to not filter the list.

I suggest to remove the lspserver.completeItemsIsIncomplete check here:
https://github.com/yegappan/lsp/blob/main/autoload/lsp/completion.vim#L538
There is already a PR to remove the check(#568), but the author deleted the fork and closed the PR.
At least we should provide an option to bypass this check.

This should fix #366