dev: Remove unused private variable in lupio_tmr.hh

This was causing the compiler-checks to fail:
https://jenkins.gem5.org/job/compiler-checks/71

Clang-11 complained of an unused private variable. This variable is
removed in this commit.

Change-Id: I4a43d3cae0fe4ffed4e1df3d94849ac7dc24cc0b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/53823
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/lupio/lupio_tmr.hh

@@ -50,7 +50,6 @@ class LupioTMR : public BasicPioDevice
     System *system;
     int nThread;
     int intType;
-    int nCPUs = 0;
 
     // Register map
     enum