Patch for src/tactic/ui/table/drop_element_wdg.py bug fetching wrong SObjects in many to many handling

Affected versions: 4.1.0v03, 4.1.0v04 are the ones I tested

How to reproduce:
To be sure there were no site or project specific interferences I dowloaded the 4.1.0.v03 VMware Image from http://community.southpawtech.com/downloads and used the VFX project.
The easiest way to show the bug is using the Shot planner. Connect to Tactic and open the VFX project, in the sidebar click on "Tools"->"Shot Planner", see the assets and shots section loading into the shot planner tab. Click on "Tools"->Shot Planner" again and keep cliking, at some point, usually after 3 to 5 reloads you'll see a duplicated asset for a specific shot into the "Asset Drop" column. The bug appears randomly every 2 to 5 reloads but gets much worse when the shots and assets number gets higher. Note that the duplicated asset is really referring to another asset, on a shot having "camera001, set001, vehicle001" but showing "camera001, camera001, vehicle001" in the Asset drop if you "click to remove" on the second camera001 it will remove set001

As oboreo suggested I'm moving the clear_cache() call from src/pyasm/biz/expression.py to src/tactic/ui/table/drop_element_wdg.py