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