cxgb4: remove unused fn to enable/disable db coalescing

Remove unused function cxgb4_enable_db_coalescing() and
cxgb4_disable_db_coalescing()

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>