factor out getDehydratedDevice

src/client.js

@@ -498,19 +498,8 @@ MatrixClient.prototype.rehydrateDevice = async function() {
         return;
     }
 
-    let getDeviceResult;
+    const getDeviceResult = this.getDehydratedDevice();
-    try {
+    if (!getDeviceResult) {
-        getDeviceResult = await this._http.authedRequest(
-            undefined,
-            "GET",
-            "/dehydrated_device",
-            undefined, undefined,
-            {
-                prefix: "/_matrix/client/unstable/org.matrix.msc2697.v2",
-            },
-        );
-    } catch (e) {
-        logger.info("could not get dehydrated device", e.toString());
         return;
     }