Remove Debug message output from UpdateMailboxContent(), it may cause Mailbox locker nested issue.

Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15025 6f19259b-4bc3-4df7-8a09-765794883524