sim: Delete the unused PowerStateDumpCallback.
sim: Delete the unused PowerStateDumpCallback.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: I8e66f31a3a6a82564d9525021ada49ce52beb1fa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32646
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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