mt76x0: remove unused mt76x0_burst_read_regs

mt76x0_burst_read_regs is not used, but keep it for eventual use. Since
we have this function now in the driver git history, we can remove it and
eventually revert this commit it the function will be needed.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>