WhiskeylakeOpenBoardPkg/PeiPolicyUpdateLib: Remove unneeded SPI header

This contents of the header file are not referenced in this
source file.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Sai Chaganty <rangasai.v.chaganty@intel.com>