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/o3/thread_context.cc

@@ -196,12 +196,6 @@ ThreadContext::pcStateNoRecord(const PCStateBase &val)
     conditionalSquash();
 }
 
-RegId
-ThreadContext::flattenRegId(const RegId& regId) const
-{
-    return cpu->isa[thread->threadId()]->flattenRegId(regId);
-}
-
 void
 ThreadContext::setMiscRegNoEffect(RegIndex misc_reg, RegVal val)
 {