Remove all kernel uses of pcb_psl, but keep in in the struct to
preserve the ABI and API for applications. It was removed in the port
to amd64, but was remained as garbage giving a micro-pessimization and
spurious single-step traps on i386.

pcb_psl was intended to be used just to do a context switch of PSL_I,
but this context switch was null in most or all versions, and
mis-switching of PSL_T was done instead.

Some history:

• in 386BSD-0.0, cpu_switch() ran at splhigh() and splhigh() did too much interrupt disabling, so interrupts were hard-disabled across cpu_switch() and too many other places
• in 386BSD-0.0-patchkit through FreeBSD-4 and FreeBSD-5 before SMPng, splhigh() did soft interrupt masking, and cpu_switch() was excessively cautious and did a cli at the start and a sti at the end to hard-disable interrupts across the switch
• SMPng replaced the spl's and cli's by spinlocks (just sched_lock?), so interrupts were hard-disabled across cpu_switch() and too many other places again
• initial attempts to fix this intended to restore some soft interrupt disabling, but to support variations in this cpu_switch() used pushfl/popfl into pcb_psl to avoid hard-coding the assumption that the initial and final states have PSL_I enabled. But the version with soft interrupt disabling wasn't used for long, or was never committed, (except I always used my different version of it for UP) so the pushfl/popl and pcb_psl to hold them have been doing less than nothing for about 14 years.