Remove time out, fixes #90

[FokkeZB]

This fixes #90, but... I now get an empty response when I send:

-- LOOP
Sending: mac tx uncnf 1 with 1 bytes
Time-out

Any idea @johanstokking?

[FokkeZB]

With this PR approved, we need some additional changes:

• [x] Remove logic that expects readLine() may return "":
  • https://github.com/TheThingsNetwork/arduino-device-lib/blob/e1bda36926ca29d277f3ba185c207eeeae42865e/src/TheThingsNetwork.cpp#L21
  • https://github.com/TheThingsNetwork/arduino-device-lib/blob/e1bda36926ca29d277f3ba185c207eeeae42865e/src/TheThingsNetwork.cpp#L94
  • https://github.com/TheThingsNetwork/arduino-device-lib/blob/e1bda36926ca29d277f3ba185c207eeeae42865e/src/TheThingsNetwork.cpp#L208
• [ ] Update #91 which also check for this:
  • https://github.com/TheThingsNetwork/arduino-device-lib/pull/91/files#diff-b2f2810008e6e975eb686e14fbd00f7fR200

@johanstokking, I'm not 100% sure I have added clearBuffer() to all and only those places where it's needed.