Clean-up redundant items: files and functions. (#2410)

Some justifications:
- power & Co: only useful on Cray, likely outdated. Use external power measurements if this is relevant.
- trace.hpp was never used and was locked_ostream's only user.

arbor/CMakeLists.txt

@@ -20,10 +20,7 @@ set(arbor_sources
     fvm_layout.cpp
     fvm_lowered_cell_impl.cpp
     hardware/memory.cpp
-    hardware/power.cpp
     iexpr.cpp
-    io/locked_ostream.cpp
-    io/serialize_hex.cpp
     label_resolution.cpp
     lif_cell_group.cpp
     cable_cell_group.cpp
@@ -50,7 +47,6 @@ set(arbor_sources
     partition_load_balance.cpp
     profile/memory_meter.cpp
     profile/meter_manager.cpp
-    profile/power_meter.cpp
     profile/profiler.cpp
     schedule.cpp
     spike_event_io.cpp

arbor/backends/gpu/fvm.hpp

@@ -1,16 +1,11 @@
 #pragma once
 
-#include <map>
 #include <string>
 
 #include <arbor/common_types.hpp>
 #include <arbor/fvm_types.hpp>
 
 #include "memory/memory.hpp"
-#include "util/rangeutil.hpp"
-
-#include "backends/event.hpp"
-
 #include "backends/gpu/gpu_store_types.hpp"
 #include "backends/gpu/shared_state.hpp"
 
@@ -20,7 +15,6 @@ namespace arb {
 namespace gpu {
 
 struct backend {
-    static bool is_supported() { return true; }
     static std::string name() { return "gpu"; }
 
     using value_type = arb_value_type;

arbor/backends/multicore/fvm.hpp

@@ -16,7 +16,6 @@ namespace arb {
 namespace multicore {
 
 struct backend {
-    static bool is_supported() { return true; }
     static std::string name() { return "cpu"; }
 
     using value_type = arb_value_type;