Return HTTP 503 from Fee Api when mempool is still syncing.

Fix for displaying git commit on About page.

frontend/src/app/interfaces/websocket.interface.ts

@@ -28,12 +28,6 @@ export interface MempoolBlock {
   index: number;
 }
 
-export interface MemPoolState {
-  memPoolInfo: MempoolInfo;
-  vBytesPerSecond: number;
-  gitCommit: string;
-}
-
 export interface MempoolInfo {
   size: number;
   bytes: number;