docs: ThreadGetMyExternalIP has been removed

It was removed in https://github.com/bitcoin/bitcoin/pull/5161
Remove misleading comment about testnet's message string.
Merge pull request #5348

be4ac91 docs: ThreadGetMyExternalIP has been removed (Pavel Vasin)
Merge pull request #5350

e0535e1 Remove misleading comment about testnet's message string. (Pavel Janík)