Remove SPARE_USRSPACE.

This constant was used to reserve space at the top of the stack to
hold translated system call arguments for non-default ABIs (the
"stackgap").  However, none of the compatibility ABIs have used the
stackgap in many years and the last use of SPARE_USRSPACE was removed
in r355373.

Reviewed by:	kib
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D22697

sys/sys/exec.h

@@ -77,7 +77,6 @@ struct execsw {
  * Prefer the kern.ps_strings or kern.proc.ps_strings sysctls to this constant.
  */
 #define	PS_STRINGS	(USRSTACK - sizeof(struct ps_strings))
-#define	SPARE_USRSPACE	4096
 
 int exec_map_first_page(struct image_params *);        
 void exec_unmap_first_page(struct image_params *);