[Qt] remove unclean net-detection in ipcParseCommandLine()

- remove the net-detection implementation in ipcParseCommandLine() as it
  will fail if the request itself is damaged or not a payment-request but
  just a normal bitcoin: URI
- try to send the received payment-request or URI via ipcSendCommandLine()
  directly to a mainnet instance and if that fails to a testnet instance
  (this should reflect normal use case of Bitcoin Core, but will cause
  problems when running both nets and playing with testnet stuff as the
  mainnet instance wont process the testnet stuff and error out)