MdePkg/ResetNotification: Rename to UnregisterResetNotify
UEFI Spec uses UnRegisterResetNotify in protocol structure
definition but uses UnregisterResetNotify in the function
prototype definition.
By searching the entire spec, Unregister* is used for
SIMPLE_TEXT_INPUT_EX_PROTOCOL.UnregisterKeyNotify(). So choose
to use UnregisterResetNotify for consistency.