packages/DOS/common.dj: remove COFF debug info generation

gcc on DOS hasn't really supported COFF-debug (-gcoff) on djgpp for a
long time.

"Sounds like the COFF debug info generation has bit-rotted in GCC.
 Nothing new here, no other platform uses COFF AFAIK."

So lets drop it too.

URL: http://curl.haxx.se/mail/lib-2013-01/0130.html