Do not set gBS to NULL when the ExitBootServices event notification for the UEFI...

Do not set gBS to NULL when the ExitBootServices event notification for the UEFI Runtime Lib is executed.

There is no guarantee that the ExitBootServices notification function from this lib will be the last ExitBootServices notification function registered by a module that uses the UEFI Runtime lib.  If additional ExitBootServices notification functions require the use of gBS, then setting to NULL will break those additional ExitBootServices notification functions.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@9982 6f19259b-4bc3-4df7-8a09-765794883524