KabylakeSiliconPkg: Remove unused PCD 'PcdUseHpetTimer'

For platforms under KabylakeOpenBoardPkg & ClevoOpenBoardPkg, both of them
are using the HPET timer unconditionally. The PCD 'PcdUseHpetTimer' is not
being used.

This commit will remove the above PCD within KabylakeSiliconPkg.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>