Update DEC file and DxeImageVerificationLib to note user that ALLOW_EXECUTE_ON_SECURITY_VIOLATION is no longer supported.

Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Dong, Guo <guo.dong@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14923 6f19259b-4bc3-4df7-8a09-765794883524