samples: mbedTLS server: Remove CONFIG_NET_SLIP_TAP assignment

Was written as "CONFIG_NET_SLIP", which doesn't exist.

Jukka Rissanen indicated in
https://github.com/zephyrproject-rtos/zephyr/pull/7810

 that
CONFIG_NET_SLIP_TAP doesn't need to be enabled here ("I prefer we remove
the line completely as the TAP is enabled automatically for qemu_x86"),
so remove the assignment rather than fixing it.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>