bhyve: Remove some no-op code for setting RIP.

fbsdrun_addcpu() read the current vCPU's RIP register from the kernel
via vm_get_register() to pass along through some layers to vm_loop()
which then set the register via vm_set_register(). However, this is
just always setting the value back to itself.

Reviewed by: corvink
Differential Revision: https://reviews.freebsd.org/D37643