signindex: do not remove index-v1.json after signing

With ~index-v2, the model is changing to offer the plain JSON file for easy
consumption.  Then gpgsign will also provide a detached PGP signature for
systems that would rather verify based on PGP signatures than JAR signatures.

!1080
!1124
#969

# Conflicts:
#	tests/signindex.TestCase

fdroidserver/signindex.py

@@ -122,7 +122,6 @@ def main():
         index_file = os.path.join(output_dir, json_name)
         if os.path.exists(index_file):
             sign_index_v1(output_dir, json_name)
-            os.remove(index_file)
             logging.info('Signed ' + index_file)
             signed.append(index_file)