ssh remoting: Do not double-register LspAdapters (#18132)

This fixes the bug with hover tooltips appearing multiple times.

Turns out everytime we receive the `CreateLanguageServer` message we'd
add a new adapter but only have a single server running for all of them.

And we send a `CreateLanguageServer` message everytime you open a
buffer.

What this does is to only add a new adapter if it hasn't already been
registered, which is also what we do locally.


Release Notes:

- N/A