completion: bash: remove old wrappers

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
completion: bash: remove zsh wrapper

It has been deprecated for one year and a half. It's time to move on.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
completion: zsh: don't hide ourselves

There's no need to hide the fact that we are on zsh any more.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
completion: remove zsh hack

We don't want to override the 'complete()' function in zsh, which can be
used by bashcomp.

Reported-by: Mark Lodato <lodato@google.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
completion: zsh: trivial cleanups

We don't need to override IFS, zsh has a native way of splitting by new
lines: the expansion flag (f).

Also, we don't need to split files by ':' or '='; that's only for words.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
completion: bash: cleanup cygwin check

Avoid Yoda conditions, use test, and cleaner statement.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>