Property 'pig.job.converted.fetch' should be unset when fetch finishes

If plan is fetchable, PIG-4171 sets

pc.getProperties().setProperty(PigImplConstants.CONVERTED_TO_FETCH, "true");

When working in grunt this session-scoped property remains set after the result is fetched. For example, if subsequent jobs aren't fetchable or the user disables fetch, this property will be still there.
We'd need to remove this property from PigContext after the fetch job finishes.