Revert "Add support for BBC DDFS disks/drives"

This reverts commit c81fcce.

DDFS was a double density DFS, deployed by Watford (and others) with
their 1770 disk interface - it's an MFM format, and could be 40 or 80
track, single or double sided (not interleaved). 80 track FM was
simply 80 track DFS format (again, single or double sided not
interleaved). Check here for more detail:
https://stardot.org.uk/forums/viewtopic.php?p=390348#p390348

src/greaseweazle/data/diskdefs.cfg

@@ -131,20 +131,6 @@ disk acorn.dfs.ss
     end
 end
 
-disk acorn.ddfs.ss
-    cyls = 80
-    heads = 1
-    tracks * ibm.fm
-        secs = 10
-        bps = 256
-        iam = no
-        gap3 = 21
-        id = 0
-        cskew = 3
-        rate = 125
-    end
-end
-
 disk acorn.dfs.ds
     cyls = 40
     heads = 2