samples: net: Fix echo_client/echo_server TLS config files

Current config files for TLS qemu_x86 contain unrecognized
`CONFIG_RAM_SIZE`, which causes cmake to fail during project generation.
They  should not be needed anyway, as qemu_x86 defaults to 8 MB of RAM.
Additionaly, echo_client crashes in this configuration with default main
stack size, hence increase it.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>