Remove unused import in Python prometheus doc (#5585)

Co-authored-by: Emídio Neto <[email protected]>

Author: mimikwang

content/en/docs/languages/python/exporters.md

@@ -213,7 +213,6 @@ from prometheus_client import start_http_server
 from opentelemetry import metrics
 from opentelemetry.exporter.prometheus import PrometheusMetricReader
 from opentelemetry.sdk.metrics import MeterProvider
-from opentelemetry.sdk.metrics.export import PeriodicExportingMetricReader
 from opentelemetry.sdk.resources import SERVICE_NAME, Resource
 
 # Service name is required for most backends