efi_loader: don't load Shim's MOK database from file

When using a file to store UEFI variables we must make sure that secure
boot related variables are not loaded from this file. With commit
9ef82e29 ("efi_loader: don't load signature database from file")
this has already been implemented for variables defined in the UEFI
specification. As most Linux distributions use Shim we should do the same
for Shim's MOK database.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>