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 nexedi/erp5!1738.

/reviewed-on nexedi/slapos!1355

stack/erp5/buildout.cfg

@@ -794,7 +794,6 @@ objgraph = 3.1.0
 oic = 0.15.1
 olefile = 0.44
 openpyxl = 2.4.8
-pandas = 0.19.2
 parso = 0.5.1
 Pillow = 6.2.2
 polib = 1.0.8