vim: Rename TabsOrSpaces to Tabs_Or_Spaces

Unfortunately we still need to keep the capitalised words.
vim: Remove tab/space detection code

This case comes up infrequently (mostly, when editing code from suckless
upstream), but it's obvious enough when presented that I'd prefer just
to do it manually.
vim: Remove misc unused settings from vimrc

I don't use these ever, really. timeoutlen was back when I was escaping
insert mode without escape, which I don't do any more... the rest are
mostly YAGNI.
vim: Show trailing whitespace, but don't remove it automatically

I have become averse to the idea of automatic whitespace removal, I
don't like my editor to touch anything without me explicitly asking me
to do so. This at least shows trailing whitespace pretty well (on my
terminal, "black" is close to, but slightly lighter than the terminal
background colour).