[Teko] Remove `amgnxn` framework

[maxfirmbach]

Description

In the near future we should think about removing the amgnxn framework. Most, if not all, functionality is already available open-source in MueLu and mostly in Teko and if not should be added there. In addition it already proves to be difficult to maintain the code, also with the transition to Tpetra.

Possible Solution and Definition of Done

Port remaining physics using amgnxn:

• elch (see Enable teko for elch problems #1130)
• elch (dofsplit)
• sti (see Introduce Teko as block preconditioner for ScaTra-Thermo-Interaction #1143)
• ssi (see Introduce Teko as block preconditioner for Solid-ScaTra-Interaction #1430)
• ssi (dofsplit)
• ssti (see Replace AMGnxn with Teko for ssti framework #1516)

Delete the amgnxn framework:

• Delete the respective code related to the amgnxn implementation (see [solver] Remove monolithic part of amgnxn #1434)
• Delete the input procedure and enum classes related to amgnxn

Interested Parties

@c-p-schmidt