Remove Default <F8> Mapping

[Jia]

Hi, I think it's better to remove the default mapping for:

1. Normally it's a convention to provide mappings that are unique to a certain plugin, is too common.
2. If has been mapped, it will be overwritten by vim-autopop8, which is undesired.
3. Provoding the API is enough, it's the user's business to decide which key to be mapped to.

[tell-k]

@js5shi

Firstly, I apologize for the delay in replying to you.

You're right, My plugin is well-behaved might be bad.
In the next version I think that it is trying to remove the default mapping.

Thanks for your suggestion.