Clear unnecessary fields after error indicators are computed

[ddundo]

With #212, error indicators are now computed at the end of each subinterval. This allows us to get rid of forward_old and adjoint_next fields after each subinterval, since they are no longer needed.

[ddundo]

Then we can also completely get rid of solutions on the enriched_mesh_seq after each subinterval, as well as the enriched mesh and function spaces, etc.

[jwallwork23]

Nice!

[ddundo]

@acse-ej321 assigning you since you said you'll do it :)

[ddundo]

@acse-ej321 @jwallwork23 do we ever need forward_old in Hessian-based adaptations? (After each subinterval's _solve_forward, I mean)