efi_loader: delete handle from events when a protocol is uninstalled

When a notification event is registered for a protocol the handle of the
protocol is added in our event notification list.  When all the protocols
of the handle are uninstalled we delete the handle but we do not remove
it from the event notification list.

Clean up the protocol removal functions and add a wrapper which
- Removes the to-be deleted handle from any lists it participates
- Remove the handle if no more protocols are present

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>