zeroinstall-injector: remove formula

The software package was renamed to "0install", and it is easier
to install it with easy_install than to update the brew formula.
(but one can still use homebrew to install python, pygtk, gnupg)
There is also a binary package available, at http://0install.net

Closes #16828.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>