SP2 Perf improvement: Remove Jahia_link checks for Content picking on each field if Content Picking module is not activated in the Jahia license

It looks likes the checks we are doing on each field loading in order to verify if there is a content picked object takes a lot of time (at least according to Serge). Here again, most of our customers do not use the CP module. So perhaps it would be easy to bypass such check if the CP module is not activated in the license key.

We could then formalize such a performance optimization in our next "Jahia 5.0 fin-tunig guide" and tell to our customers, even the ones which baught some professional or enterprise editions (which by default include the CP module), to ask for a custom license key without such a module for performance reasons, if they do not (plan to) use it (at least for the next months).

If we can make something better, fine... but at least such workarounds could problably be done in a couple of hours without heavy refactorings (and then the risk to introduce new bugs)