removed mutable old-solution from ProblemInstat

4 jobs for issue/problem_instat_oldsolution in 26 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #8697
debian:10 clang-6-libcpp-17

00:26:02

passed #8696
debian:10 gcc-8-17

00:18:02

passed #8698
debian:9 gcc-6-14

00:19:33

passed #8699
ubuntu:18.04 clang-6-17

00:24:30