Remove "__brew_ps1" function from completion

This was probably silly and I'm probably the only person that ever used
it. I still do, actually, but it's not really a completion function,
it's simple enough to just stick in a shell startup script, and removing
it makes the completion script usable under the new dynamic loading
scheme provided by bash-completion 1.99+.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>