cpu: Delete an unnecessary return in RegId::flatIndex.

panic() is marked [[noreturn]], so there's no need to put a dummy return
after it.

Change-Id: Icebba480ef583a149188d4d6f9481fe0bbd95082
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45225
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/cpu/reg_class.hh

@@ -192,7 +192,6 @@ class RegId
             return Scale * regIdx + elemIdx;
         }
         panic("Trying to flatten a register without class!");
-        return -1;
     }
     /** @} */