powerpc/configs/6xx: Drop no longer needed CONFIG_BT_HCIUART_H4

Since commit 943cc592 ("Bluetooth: bpa10x: Use h4_recv_buf helper
for frame reassembly") we no longer need to set CONFIG_BT_HCIUART_H4
in our defconfigs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>