remove mention of EOL CDBS

Dear maintainer, there are only an handfull users of CDBS + autotools left as of today. (out of the complete list hereunder)

Here is alreadt a MR than cleanse autotools-dev documention.

algol68g                                1114785
antlr3                                  1091780
basilisk2                               
console-data                            1091781
debian-design                           
debian-parl                             
edac-utils                              1113952
etoys                                   
fonts-noto                              
fortunes-br                             1115174
gnome-system-tools                      1099822
guile-cairo                             1115100
ibus-client-clutter                     1115101
impressive-display                      1115866
jack-tools                              
kawari8                                 1099821
libexttextcat                           1106169
libgdchart-gd2                          
lightdm-autologin-greeter               1115867
lightdm-remote-session-x2go             1115099
madplay                                 1114981
munin-libvirt-plugins                   1099809
openboard-extras-nonfree                1115868
reniced                                 1115176
slirp                                   1115159
squeak-vm                               
st                                      
standardskriver                         1115869
streamripper                            1114788
tarantool                               1099946
tthsum                                  1114786
tv-fonts                                1111912
vde2                                    1114784