Remove SPARE_USRSPACE.

[jhb]

Summary

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.

Test Plan

• grep -r doesn't turn up any instances in the kernel
• amd64, i386, and aarch64 kernels build