Remove [Listen warning] prefix from warning message

Since Listen.logger.warn formats the error message with a `WARN -- : #{msg}`
we don't need the [Listen warning] part as it is redundant

lib/listen/adapter.rb

@@ -36,7 +36,7 @@ def _usable_adapter_class
 
       def _warn_polling_fallback(options)
         msg = options.fetch(:polling_fallback_message, POLLING_FALLBACK_MESSAGE)
-        Listen.logger.warn "[Listen warning]:\n  #{msg}" if msg
+        Listen.logger.warn(msg) if msg
       end
     end
   end

spec/lib/listen/adapter_spec.rb

@@ -49,7 +49,7 @@
 
       it 'warns polling fallback with default message' do
         msg = described_class::POLLING_FALLBACK_MESSAGE
-        expect(Listen.logger).to receive(:warn).with("[Listen warning]:\n  #{msg}")
+        expect(Listen.logger).to receive(:warn).with(msg)
         Listen::Adapter.select
       end
 
@@ -59,7 +59,7 @@
       end
 
       it 'warns polling fallback with custom message if set' do
-        expected_msg = "[Listen warning]:\n  custom fallback message"
+        expected_msg = "custom fallback message"
         expect(Listen.logger).to receive(:warn).with(expected_msg)
         msg = 'custom fallback message'
         Listen::Adapter.select(polling_fallback_message: msg)