base-stats: Fix Watt Unit

Watt had two implementations. Since having the unit
printed as Watt is more relevant than as Joule/Second,
keep the class.

Change-Id: Ic9ae755115e2eca94492f3d5b11245db9fe42bb6
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43006
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>