Removed a degeneracy check from the HLLD and LHLLD Riemann Solvers.

[tomidakn]

It turned out that we should not remove the double-star states even when the normal magnetic field bx is very weak. If this branch is selected when bx is actually non-zero, it may return unphysical flux. In fact, this check is not mentioned in Miyoshi & Kusano 2005 but already existed in the old Athena,

I do not think this has affected many people, as this is problematic only when the magnetic fields are very weak and the flow is not smooth. The probability that this branch is selected is very low in a realistic flow.

Thanks to Kazunari Iwasaki.

Prerequisite checklist

• [X] My code follows the Athena++ Style Guide
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation in the Wiki accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.