cpu: Delete an unnecessary return in RegId::flatIndex.
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 <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5