ice_dyn_vp: remove 'picard_solver' subroutine

The 'picard_solver' subroutine has been abandoned for quite a while
and the Picard algorithm can be used by using the Anderson solver
with `im_andacc = 0`, i.e. not saving any residuals.

Remove the subroutine and move the logic around the namelist variable
'algo_nonlin' from 'imp_solver' to 'anderson_solver'.

This way, the new developments made in 'anderson_solver' (using the FGMRES
solver from GEM, choosing the orthogonalization method, the addition of the
precondition subroutine) can also be used with `algo_nonlin = 1`.