Deprecate/remove parts of the legacy Analysis Server protocol that are not used by clients

[DanTup]

There are parts of the legacy server protocol that are only used by VS Code. VS Code has been using LSP for all recent versions of the SDK and all code related to the legacy protocol is being removed for the next version of the extension. We should deprecate and/or remove parts of the legacy protocol that are not used by IntelliJ (or any other clients we know of).

Some possible candidates:

• Occurrences (IntelliJ doesn't use these)
• Signature Help (I believe this was added specifically for VS Code)

Although, I don't know if there's a simple way to determine what IntelliJ does/doesn't use.

[bwilkerson]

@jwren What's the easiest way for someone (not necessarily you) to find out which protocols are being used in the versions of IntelliJ and Android Studio that we currently support?