Remove FESolver and derived types

[fverdugo]

FESolver et al. are no needed and can be removed to simplify the code.

Since PR #705 it is not needed to use FE solvers in the user code i.e.

op = AffineFEOperator(...)
solver = LinearFESolver(LUSolver())
uh = solve(solver,op)

Can be written as

op = AffineFEOperator(...)
solver = LUSolver()
uh = solve(solver,op)

We can also remove them from the library completely in the next breaking release.

[fverdugo]

To check if removing of FESolver will cause trouble in GridapODEs cc @oriolcg

[oriolcg]

To check if removing of FESolver will cause trouble in GridapODEs cc @oriolcg

It should be fine. In GridapODEs we have TransientFESolver that receives an ODESolver. Each ODESolver calls solve! using a NonlinearSolver so no FESolver involved. In any case, we could also remove the TransientFESolver there to keep things aligned with Gridap. I'll take care of that.