Revert "stack/erp5: fix pandas version in erp5"

This reverts commit ca5ba490.

It's actually a good idea to update our pandas version (0.19.x is
already 7 years old, in one project we recently run into a bug of this
pandas version).

We only need to prepare erp5 with few changes so that it also support
those newer versions. This preperation was done in erp5@11b22568.

stack/erp5/buildout.cfg

@@ -869,7 +869,6 @@ strict-rfc3339 = 0.7
 webcolors = 1.10
 rfc3987 = 1.3.8
 jsonpointer = 2.2
-pandas = 0.19.2
 numpy = 1.13.1
 scipy = 0.19.0
 ZConfig = 2.9.3