UefiCpuPkg/MpInitLib: Remove redundant CpuStateFinished State.

UefiCpuPkg/MpInitLib: Remove redundant CpuStateFinished State.

Current CPU state definition include CpuStateIdle and CpuStateFinished.
After investigation, current code can use CpuStateIdle to replace the
CpuStateFinished. It will reduce the state number and easy for maintenance.

> Before this patch, the state transitions for an AP are:
>
>   Idle ----> Ready ----> Busy ----> Finished ----> Idle
>        [BSP]       [AP]       [AP]           [BSP]
>
> After the patch, the state transitions for an AP are:
>
>   Idle ----> Ready ----> Busy ----> Idle
>        [BSP]       [AP]       [AP]

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>