Remove unused members of struct lndstr, lonstr, nukstr, trtstr

struct lndstr member lnd_flags is a leftover from Empire3's C_SYNC,
which was ripped out in 4.0.0.

struct lonstr member l_sel, struct nuk_str members nuk_ship, nuk_land,
and struct trtstr member trt_bond have been there basically forever
without any use.

include/treaty.h

@@ -50,7 +50,6 @@ struct trtstr {
     char trt_fill;
     short trt_acond;		/* conditions for proposer */
     short trt_bcond;		/* conditions for accepter */
-    float trt_bond;		/* amount of bond involved (unused) */
     time_t trt_exp;		/* expiration date */
 };