Do not check Elixir version in the Fly runtime (#2710)

lib/livebook/runtime/fly.ex

@@ -296,19 +296,14 @@ defmodule Livebook.Runtime.Fly do
   defp fetch_runtime_info(child_node) do
     # Note: it is Livebook that starts the runtime node, so we know
     # that the node runs Livebook release of the exact same version
+    #
+    # Also, the remote node already has all the runtime modules in
+    # the code path, compiled for its Elixir version, so we don't
+    # need to check for matching Elixir version.
 
-    %{
+    %{pid: pid} = :erpc.call(child_node, :persistent_term, :get, [:livebook_runtime_info])
-      pid: pid,
-      elixir_version: elixir_version
-    } = :erpc.call(child_node, :persistent_term, :get, [:livebook_runtime_info])
 
-    if elixir_version != System.version() do
+    {:ok, pid}
-      {:error,
-       "the local Elixir version (#{inspect(System.version())}) does not" <>
-         " match the one used by the runtime (#{elixir_version})"}
-    else
-      {:ok, pid}
-    end
   end
 
   defp initialize_node(child_node) do

rel/server/overlays/bin/start_runtime.exs

@@ -13,8 +13,7 @@ node = :"#{node_base}@#{machine_id}.vm.#{app}.internal"
 
 # We persist the information before the node is reachable
 :persistent_term.put(:livebook_runtime_info, %{
-  pid: self(),
+  pid: self()
-  elixir_version: System.version()
 })
 
 Application.put_env(:kernel, :inet_dist_listen_min, dist_port)