DISCO_L475VG_IOT01A: remove rtos from build_travis.py

Author: bcostm

tools/build_travis.py

@@ -236,7 +236,6 @@
     {"target": "DISCO_L475VG_IOT01A",
      "toolchains": "GCC_ARM",
      "tests": {""     : ["MBED_2", "MBED_10", "MBED_11", "MBED_16"],
-               "rtos" : ["RTOS_1", "RTOS_2", "RTOS_3"],
                "usb"  : ["USB_1", "USB_2" ,"USB_3"],
                }
      },