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/thread_context.hh

@@ -219,8 +219,6 @@ class ThreadContext : public PCEventScope
 
     virtual void setMiscReg(RegIndex misc_reg, RegVal val) = 0;
 
-    virtual RegId flattenRegId(const RegId& reg_id) const = 0;
-
     // Also not necessarily the best location for these two.  Hopefully will go
     // away once we decide upon where st cond failures goes.
     virtual unsigned readStCondFailures() const = 0;