Do not generate the extra output files index-intro.html and index-news.html.

Fix the change https://android-review.googlesource.com/#/c/51722/

Change-Id: Ie34b7609723bcb2a2c58f1fca1d91a718eecd1c9
diff --git a/scripts/build.py b/scripts/build.py
index f33fe0c..2600f92 100755
--- a/scripts/build.py
+++ b/scripts/build.py
@@ -104,11 +104,13 @@
 
   if 'index-intro.md' in files:
     intro = md(os.path.join(curdir, 'index-intro.md'))
+    del files[files.index('index-intro.md')]
   else:
     intro = ''
 
   if 'index-news.md' in files:
     news = md(os.path.join(curdir, 'index-news.md'))
+    del files[files.index('index-news.md')]
   else:
     news = ''