[solver] Reduce linear solver related input file parameters

[maxfirmbach]

Description

Both the variables AZCONV and AZSUB are rarely manually steered/used, with the default values being already great choices. I think we should remove both of them. If one really needs to specify them, it is already possible by giving a linear solver config file.

Possible Solution and Definition of Done

• Remove the input parameter AZCONV from the code base, test cases and tutorials.
• Remove the input parameter AZSUB from the code base, test cases and tutorials.
• Make things work and ctest.

Interested Parties

@mayrmt @reginabuehler (I put you both here as this is more for user experience / ease of usage e.g. maybe tutorial related)

[mayrmt]

I'd even be fine to push towards specifying the iterative solver via the configuration file only.

[maxfirmbach]

@mayrmt I can open a new issue for that, describing the long term effort / vision for the solver / preconditioner input procedure.