[DCCP]: Remove unused inline function

The function follows48(), which is a special-case of dccp_delta_seqno(),
is nowhere used in the DCCP code, thus removed by this patch.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>