Bluetooth: Remove unnecessary bt_dev_esco struct

This information should be part of the main BR/EDR context struct,
rather than there being a separate member in struct bt_dev. If/when
the needed ESCO information grows we can consider having a separate
struct, but even then it should be part of the main BR/EDR struct
instead of sitting directly in bt_dev.

Change-Id: I3edf120606ea6c6974f515bba90de2b25fc6fac6
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>