Remove secp256k1_fe_const_b

It's last use was eliminated in #1217.

Author: roconnor-blockstream

src/group_impl.h

@@ -73,8 +73,6 @@ static const secp256k1_ge secp256k1_ge_const_g = SECP256K1_G;
 #endif
 /* End of section generated by sage/gen_exhaustive_groups.sage. */
 
-static const secp256k1_fe secp256k1_fe_const_b = SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, SECP256K1_B);
-
 static void secp256k1_ge_set_gej_zinv(secp256k1_ge *r, const secp256k1_gej *a, const secp256k1_fe *zi) {
     secp256k1_fe zi2;
     secp256k1_fe zi3;