lefthook unistall does not remove lefthook.checksum

[JuliusHenke]

I found a bug in the lefthook uninstall command. It does not remove the .git/info/lefthook.checksum file.

This is a problem since following order of commands results in no hooks being installed into .git/hooks:

1. lefthook install
2. lefthook uninstall
3. lefthook install

I would like to try to fix this with a PR, if you are cool with it? 😃

[mrexox]

@JuliusHenke yeah! This would be nice! I'll be waiting for a PR :)