Front derailleur won't spring back to lower gears
If you believe you have diagnosed the problem then a clean and a service should be all that is required. You don't have to remove the FD.

WD-40 will not displace mud effectively. I would: 


use a hose with a decent amount of pressure to remove the gunk, wait for it to dry. (be careful not to spray around the bottom bracket)
apply WD-40 to any moving parts on the derailleur but focus on the pivot points to remove old lube. 
once the WD-40 has evaporated apply some decent chain lube to the pivots.


If this doesn't fix the problem and the cable is moving freely the derailleur may be bent or the spring has lost the required tension.