proc: Remove kernel stack swapping support, part 6

• Remove most checks of the P_INMEM flag.
• Some uses remain since a few userspace tools, e.g., ps(1) and top(1) expect the flag to be set. These can be cleaned up but the code has most likely been copy-pasted elsewhere and while linger for a long time.

Tested by: pho
Reviewed by: alc, imp, kib
Differential Revision: https://reviews.freebsd.org/D46117