Changed the way forces on Drudes are calculated.

Simplified the function call slightly; with SCF, we call
relax_shell_flexcon() as before, but with Lagrangian,
we just fall back to the normal call to do_force(), which is
what was left in relax_shell_flexcon() anyway.

Change-Id: I144e2da9eebe3378b2c006962b0079d6bba7e5fb