Remove wait_for_key_rotation_ms config option

I learned that makeKeyDelay has been deprecated in matrix-js-sdk ever since the switch to the new encryption manager, so this config option has no effect.

src/config/ConfigOptions.ts

@@ -207,12 +207,6 @@ export interface ConfigOptions {
    * Take care when changing these options.
    */
   matrix_rtc_session?: {
-    /**
-     * How long (in milliseconds) to wait before rotating end-to-end media encryption keys
-     * when someone leaves a call.
-     */
-    wait_for_key_rotation_ms?: number;
-
     /**
      * How long we wait before retrying after a network error on any of the requests.
      */