Bluetooth: host: Use CONFIG_BT_L2CAP_RX_MTU always to set L2CAP RX MTU

Remove dependency on CONFIG_BT_ACL_FLOW_CONTROL and use
CONFIG_BT_L2CAP_RX_MTU always to set L2CAP RX MTU.
The ATT MTU is set from two different KConfig options depending
on if CONFIG_BT_ACL_FLOW_CONTROL is enabled, which makes it
a confusing option and hard to provide a conf file that
supports multiple board configurations.

This changes the behavior when CONFIG_BT_ACL_FLOW_CONTROL and
CONFIG_BT_BUF_RX_LEN was used to set the L2CAP RX MTU, and by
extension the ATT MTU.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>