samples: smp_svr: Fix configuration when BT_HCI_ACL_FLOW_CONTROL is on

Fix smp_svr Bluetooth configuration for when BT_HCI_ACL_FLOW_CONTROL
option has been enabled. In this case the CONFIG_BT_L2CAP_RX_MTU
is used instead of CONFIG_BT_RX_BUF_LEN to set the RX MTU.
This lead to a negotiated MTU of 23, which is not supported by
the SMP_SVP protocol.

Removed the board specific configuration files as these are just
duplications of the overlay-bt.conf or overlay-bt-tiny.conf and
the readme specifies how these should be used.

Reduced the TX MTU to end up with the same value as the RX MTU,
CONFIG_BT_RX_BUF_LEN - 4 (HCI ACL header) - 4 (L2CAP header).

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