lib: efi_loader: don't delete invalid handles

Change efi_delete_handle() to not free EFI handles twice.

This change tries to resolved an issue seen since U-Boot v2022.07
in which ExitBootService() attempts to release some EFI handles twice.

The issue was seen booting a EFI shell that invokes 'connect -r' and
then boots a Linux kernel. Execution of connect command makes EFI
subsystem to bind a block device for each root block devices EFI handles.
However these EFI device handles are already bound to a driver and we
can have 2 registered devices relating to the same EFI handler. On
ExitBootService(), the loop removing the devices makes these EFI handles
to be released twice which corrupts memory.

This patch prevents the memory release operation caused by the issue but
but does not resolve the underlying problem.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>

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