Restore a part of r323722.

Do not return from interrupt using the POP_FRAME;iret instruction
sequence, always jump to doreti.

The user segments selectors saved on the stack might become invalid
because userspace manipulated LDT in a parallel thread. trap() is
aware of such issue, but it is only prepared to handle it at iret and
segment registers load operations in doreti path.

Also remove POP_FRAME macro because it is no longer used.

Reviewed by: bde, jhb (as part of r323722)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week