Xpra: Ticket #534: old unsupported code removal

This is a tracker ticket for changes already committed.

None of these changes should have any effect on clients or servers as far back as v0.3.x (the oldest version that can connect to trunk client / servers).

Features now assumed:

• r5819: "compressible_cursors"
• r5813 + r5818 : "window_configure" which allows us to remove:
  • r5814: "move-window" and "resize-window" packet handlers
• r5817: "chunked_compression"

Note: even though the feature flags are no longer used, they are still advertised for backwards compatibility.