net: openthread: Fix OPENTHREAD_CONFIG_TCP_ENABLE redefinition warning

OPENTHREAD_CONFIG_TCP_ENABLE is now set by the OpenThread build system,
based on the CONFIG_OPENTHREAD_CLI_TCP_ENABLE Kconfig entry.
Setting this symbol in the config file is therefore redundant, and
causes multiple build warnings.

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