Purge all legacy interface configurations so there's no more confusion
over which one to use.

Also remove doc/INSTALL.txt which mentions ft2232 but otherwise just
duplicates what INSTALL says.

Change-Id: Ic94f808f123d4917e600b79309f1272c78a7bb11
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3236


Tested-by: jenkins
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>