vim: Assume that autocmd support is always built-in

I cannot recall when it was the last time I saw a version of Vim built
without support for "autocmd". Just remove the conditional checking for
it and assume that things will work.
vimrc: Do not highlight EOL spaces in cursor line

Spaces at the end of the line in insert mode are not highlighted now at the
line that contains the cursor. This is good because it prevents the matches
to appear and disappear in quick progression while writing, which can be
visually annoying. A couple of autocommands are used to enable highlighting
also the cursor line when exiting insert mode.