Remove link from how_to_api.md

Remove link to TCP tutorial, which no longer exists.

Author: Amanda Butler

docs/tutorials/using_apis/how_to_api.md

@@ -4,7 +4,6 @@ This section contains step-by-step tutorials that show you how to perform specif
 
 ### Connectivity
 
-- [TCP sockets](cellular-tcp-sockets.html).
 - [Light control using Mesh](../tutorials/light-control.html).
 - [BLE](ble-tutorial.html).
 - [LoRa Network](LoRa-tutorial.html).