cxgbe: delete now-redundant vnet decls

r324539 gathered some vnet decls into netinet/tcp_var.h, so that they
are now redundant in dev/cxgbe/tom/{t4_cpl_io.c,t4_ddp.c}. This triggers
gcc -Wredundant-decls.

Reviewed by: np
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12674