As discussed in #10749, I suggested that we remove FTP support to reduce possible attack surface
as there are no Alpine mirrors which use FTP.

There are also no known mirrors of any other apk-using distribution which use FTP, so I believe
we are safe to remove support for FTP-based mirrors in apk-tools 3.

Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>