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/loan.h

@@ -50,7 +50,6 @@ struct lonstr {
     natid l_loner;		/* loan shark */
     natid l_lonee;		/* sucker */
     signed char l_status;	/* loan status */
-    short l_sell;		/* pointer to trade file (unused) */
     int l_irate;		/* interest rate */
     int l_ldur;			/* intended duration */
     long l_amtpaid;		/* amount paid so far */