Bluetooth: drivers: Remove unmaintained B91 HCI driver

The driver isn't currently buildable due to "west blobs" support never
having been added for hal_telink. Furthermore, even if the blob
dependency is manually made available it turns out the code has
bitrotten to the point where it doesn't build anymore. This situation
has continued for several years without anyone taking action, so I think
it's safe to assume this is unmaintained and should be removed.

Signed-off-by: Johan Hedberg <johan.hedberg@gmail.com>