Fix the following gcc warnings on sparc64:
sys/netinet/tcp_stacks/fastpath.c:127: warning: redundant redeclaration of 'tcprexmtthresh' [-Wredundant-decls] sys/netinet/cc/cc.h:60: warning: previous declaration of 'tcprexmtthresh' was here
It looks like tcprexmtthresh is already defined in cc.h, no need to
redeclare it in fastpath.c.