cpu: Get rid of the unused o3::ThreadContext::readReg.

Change-Id: I5c654240d7ccb09f983b4e87dde15e317fb7d953
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49106
Maintainer: Gabe Black <gabe.black@gmail.com>
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.hh

@@ -178,12 +178,6 @@ class ThreadContext : public gem5::ThreadContext
 
     /** Reads an integer register. */
     RegVal
-    readReg(RegIndex reg_idx)
-    {
-        return readIntRegFlat(flattenRegId(RegId(IntRegClass,
-                                                 reg_idx)).index());
-    }
-    RegVal
     readIntReg(RegIndex reg_idx) const override
     {
         return readIntRegFlat(flattenRegId(RegId(IntRegClass,