Remove caching of user retrieval

The caching of the load_user function already created several bugs
(INFRA 908/1018).
It requires to think about adding (or deleting in the admin view)
the current_user to the session. This is easy to miss.

It saves some database queries but we don't need this optimization.
Performances are not impacted and are good enough so far.

Removing the caching makes the application more robust and easier to
maintain.

JIRA INFRA-1025 #action In Progress