Revert "Add GET endpoints for getting the state of the finder"

This reverts commit 750b2a6313.

This can cause a deadlock if the endpoints are accessed in the core event
loop via execute action.

  fzf --listen 6266 --bind 'space:execute:curl localhost:6266'

Technically, there's no reason to use the API because the information is
already available via `{}` and `{q}`, but I'd like to completely remove
the risk of misuse.

src/options.go

@@ -113,7 +113,7 @@ const usage = `usage: fzf [options]
     --read0                Read input delimited by ASCII NUL characters
     --print0               Print output delimited by ASCII NUL characters
     --sync                 Synchronous search for multi-staged filtering
-    --listen=HTTP_PORT     Start HTTP server to receive actions
+    --listen=HTTP_PORT     Start HTTP server to receive actions (POST /)
     --version              Display version information and exit
 
   Environment variables