only compute hydro version of shock variable for hybrid Riemann

[zingale]

in the CTU solver, we compute a local version of the shock flag used just for the Riemann solver. This removes the SHOCK_VAR check, since we only need this local version if we are doing the hybrid Riemann solve. This local shk array is never used for the StateData USHK flag.

PR summary

PR motivation

PR checklist

• [ ] test suite needs to be run on this PR
• [ ] this PR will change answers in the test suite to more than roundoff level
• [ ] all newly-added functions have docstrings as per the coding conventions
• [ ] the CHANGES file has been updated, if appropriate
• [ ] if appropriate, this change is described in the docs

[zingale]

this fixes a coverity dead code issue.