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/nuke.h

@@ -58,9 +58,7 @@ struct nukstr {
     short nuk_mission;		/* mission code, unused */
     short nuk_radius;		/* mission radius, unused */
     /* end of part matching struct empobj */
-    short nuk_ship;		/* currently aboard ship (unused) */
     short nuk_plane;		/* currently aboard plane */
-    short nuk_land;		/* currently aboard land (unused) */
 };
 
 struct nchrstr {