Remove SystemLayerImplLibevent (#19396)

#### Problem

The libevent-based implementation of `System::Layer` is unused, untested
in CI, and probably stale. (It was originally written as a sanity check on
System::Layer APIs, chosen as a representative substrate independent of
any active platform.)

#### Change overview

Remove `SystemLayerImplLibevent`.

#### Testing

CI to confirm build edits.

scripts/tools/check_includes_config.py

@@ -111,9 +111,6 @@
     'src/lib/support/CHIPListUtils.h': {'set'},
     'src/platform/DeviceSafeQueue.h': {'queue'},
 
-    # libevent itself is unsuitable for small platforms.
-    'src/system/SystemLayerImplLibevent.h': {'list', 'vector'},
-
     # Only uses <chrono> for zero-cost types.
     'src/system/SystemClock.h': {'chrono'},
     'src/platform/mbed/MbedEventTimeout.h': {'chrono'},

src/system/BUILD.gn

@@ -205,9 +205,6 @@ static_library("system") {
 
   if (chip_system_config_use_sockets) {
     sources += [ "SocketEvents.h" ]
-    if (chip_system_config_event_loop == "Libevent") {
-      libs = [ "event" ]
-    }
   }
 
   if (chip_with_nlfaultinjection) {