net/tls: Remove the __TLS_DEC_STATS() macro.

The commit d26b698d ("net/tls: add skeleton of MIB statistics")
introduced __TLS_DEC_STATS(), but it is not used and __SNMP_DEC_STATS() is
not defined also. Let's remove it.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>