Readme.md: Remove PurleyOpenBoardPkg

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2322



Removes all references to PurleyOpenBoardPkg in Readme.md files in
edk2-platforms as the package will be removed in a future change.

Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Shifei A Lu <shifei.a.lu@intel.com>
Cc: Isaac W Oram <isaac.w.oram@intel.com>
Cc: Xiaohu Zhou <bowen.zhou@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>