Remove OOM error handling in khash.h

- removed OOM error handling in ```kh_resize```
- changed return type of ```kh_resize``` to ```void```
- removed checks of ```kh_resize``` return value

I think it is needed because ```kh_resize``` is exported by ```khash.h``` so its return value can be misused in nvim code. But if we don't want to do such changes in klib then feel free to close this PR.