perf/x86/cstate: Remove unused 'struct perf_cstate_msr'

Use of this structure was removed in:

  8f2a28c5 ("perf/x86/cstate: Use new probe function")

Remove the now stale type as well.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>