Disable AMG parallel index set for non AMG solver

This line seems to be there for historical reasons. Now there seems to be another class dedicated to AMG solvers.
2 jobs for support/dune-copasi-latest in 7 minutes and 39 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Test
failed debian:10--clang #235443
cores:4 mem:16G

00:06:09

failed debian:10--gcc #235442
cores:4 mem:16G

00:07:39

 
Name Stage Failure
failed
debian:10--clang Test
Errors while running CTest
Terminating dunecontrol due to previous errors!
+ exit 1
Uploading artifacts for failed job
Uploading artifacts...
junit/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=235443 responseStatus=201 Created token=dyeFVSS9
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
debian:10--gcc Test

JUnit report for CTest results written to /builds/copasi/dune-pdelab/junit/dune-pdelab-cmake.xml
--- Failed to build dune-pdelab ---
Uploading artifacts for failed job
Uploading artifacts...
junit/*.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... ok
id=235442 responseStatus=201 Created token=PCSd6pqQ
Cleaning up file based variables
ERROR: Job failed: exit code 1