vim: set g:mapleader instead of remapping <space>

This is technically the correct way, I used the remap before to support
both <space> and the default \ as leader keys.

Remapping interferes with nvim's which-key though. It's just a warning
and I'm not sure if it actually causes features to break. I'm only
setting mapleader for now to silence this.

I think the mapping was also convenient to be able to map \<key> ad hoc
rather than use <space><key> or something. We'll deal with this later.