Removed web thing appears in Add Thing screen

[mrstegeman]

(Originally from: https://github.com/mozilla-iot/gateway/issues/855)

STR:

• Add a web thing by its URL
• Remove the thing
• Click add button

Expected:

• Thing doesn't appear in add thing screen unless it is broadcasting via mDNS or Bluetooth

Actual:

• Removed thing appears in Add Thing screen