Remove references to obsolete index from requirements.txt

samples/asteroids/requirements.txt

@@ -1,2 +1 @@
---pre --extra-index-url https://archive.panda3d.org/branches/deploy-ng
 panda3d

tests/requirements.txt

@@ -1,3 +1,2 @@
 pytest==3.2.0
---pre --extra-index-url https://archive.panda3d.org/branches/deploy-ng
 panda3d