net: remove broken net_set_boot_mask() boot device validation

There are many problems with net_set_boot_mask():

1) It is broken when using the device model instead of "-net nic". Example:
   $ qemu-system-x86_64 -device rtl8139,vlan=0,id=net0,mac=52:54:00:82:41:fd,bus=pci.0,addr=0x4 -net user,vlan=0,name=hostnet0 -vnc 0.0.0.0:0 -boot n
   Cannot boot from non-existent NIC
   $
2) The mask was previously used to set which boot ROMs were supposed to be
   loaded, but this was changed long time ago. Now all ROM images are loaded,
   and SeaBIOS takes care of jumping to the right boot entry point depending on
   the boot settings.
3) Interpretation and validation of the boot parameter letters is done on
   the machine type code. Examples: PC accepts only a,b,c,d,n as valid boot
   device letters. mac99 accepts only a,b,c,d,e,f.

As a side-effect of this change, qemu-kvm won't abort anymore if using "-boot n"
on a machine with no network devices. Checking if the requested boot device is
valid is now a task for the BIOS or the machine-type code.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
