fastmodel,cpu: Remove the flattenRegId ThreadContext method.

This is no longer used or necessary.

Change-Id: Ide8dd74b5d39b245d3d71979dd84c7fee60d566e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51234
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/cpu/simple_thread.hh

@@ -290,12 +290,6 @@ class SimpleThread : public ThreadState, public ThreadContext
         return isa->setMiscReg(misc_reg, val);
     }
 
-    RegId
-    flattenRegId(const RegId& regId) const override
-    {
-        return isa->flattenRegId(regId);
-    }
-
     unsigned readStCondFailures() const override { return storeCondFailures; }
 
     bool