cpu: Get rid of the unused NumRegClasses constant.

Change-Id: Icb2918a97f58a79159307a5d7cba877899268caa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45227
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/cpu/reg_class.hh

@@ -77,12 +77,6 @@ class RegClassInfo
     RegIndex zeroReg() const { return _zeroReg; }
 };
 
-/** Number of register classes.
- * This value is not part of the enum, because putting it there makes the
- * compiler complain about unhandled cases in some switch statements.
- */
-const int NumRegClasses = MiscRegClass + 1;
-
 /** Register ID: describe an architectural register with its class and index.
  * This structure is used instead of just the register index to disambiguate
  * between different classes of registers. For example, a integer register with