Fix the #include mess in solvers.hh

Sort #include statements and remove unnecessary ones.

enabled an automatic merge when the pipeline for c7e8044e succeeds

canceled the automatic merge

Uhm, one commit message refers to solver.hh, not solvers.hh

added 2 commits

• 5b2664ca - [solvers.hh] Clean up the #include-mess.
• 42acfa9a - [solvers.hh] Remove unused #include directives.

Compare with previous version

enabled an automatic merge when the pipeline for 42acfa9a succeeds

Force-pushed a fixed commit message.

merged

mentioned in commit ba9e6e7f

mentioned in commit 49607b28

mentioned in merge request !140 (merged)