Unmap mapping in Select mode so it doesn’t interfere

`map` and `vmap`, surprisingly, apply not only to Visual mode but also to [Select mode](http://vimdoc.sourceforge.net/htmldoc/visual.html#Select). (See [`:help mapmode-s`](http://vimdoc.sourceforge.net/htmldoc/map.html#mapmode-s).) However, this doesn’t make sense if the mapping contains a printable character, which it often does – it stops the user from typing that character and seeing the result immediately. So you have to use `sunmap` to remove it (or use `xmap` instead of `vmap`.)