[DCCP] timewait: Remove leftover extern declarations

Gerrit Renker noticed dccp_tw_deschedule and submitted a patch with a FIXME,
but as he suggests in the same patch the best thing is to just ditch this
declaration, while doing that also noticed that tcp_tw_count is as well not
defined anywhere, so ditch it too.

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>