remove unused spans list on Transaction object (#387)

this should allow Span objects to be collected a bit earlier by the GC

closes #387

Author: beniwohli

elasticapm/traces.py

@@ -38,7 +38,6 @@ def __init__(self, tracer, transaction_type="custom", trace_parent=None, is_samp
         self.transaction_type = transaction_type
         self._tracer = tracer
 
-        self.spans = []
         self.dropped_spans = 0
         self.context = {}
         self.tags = {}
@@ -80,7 +79,6 @@ def end_span(self, skip_frames):
             span.frames = self._tracer.frames_processing_func(span.frames)[skip_frames:]
         else:
             span.frames = None
-        self.spans.append(span)
         set_span(span.parent)
         self._tracer.queue_func(SPAN, span.to_dict())
         return span