Remove unused WebView import

Prevent build failure. 
Reference: philipphecht#133

Author: wangwcq

android/src/main/java/com/philipphecht/RNDocViewerModule.java

@@ -9,7 +9,6 @@
 import com.facebook.react.bridge.ReactMethod;
 import com.facebook.react.bridge.ReadableMap;
 import com.facebook.react.bridge.ReadableArray;
-import com.facebook.react.views.webview.ReactWebViewManager;
 
 /* bridge react native
 int size();