[v9] hotfix: fix new implementation of concrete_getu

[AayushSabharwal]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[AayushSabharwal]

Tests pass and this hotfix is important so I'll remove the accidentally committed src/problems/linearproblem.jl and merge

[SebastianM-C]

can we get a new backport release with this?

[AayushSabharwal]

#3696 is almost done, I'll tag once it merges