libfetch: remove support for FTP
As discussed in #10749 (closed), 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