[Developers] Deleting deprecated property /chemistry/chemical_element/atomic_mass_old

[Bryan Cheung]

https://bugs.freebase.com/browse/DA-728

This property has been superceded by the current atomic_mass  
property.  If anyone is still using this property for any reason,  
please update your queries to use the atomic_mass property.  I'd like  
to delete this property by the end of the week, unless someone needs  
more time to update their queries.

Bryan