Removed Spanner.get_timespan() method.

Use inspect(spanner).get_timespan() instead.

CHANGE: should be noted in the 2.15 release notes' summary of spanner changes.
From the users' perspective the change will mean only that inspect() knows what
to do with spanners.

MODEL: this commit marks the first time the InspectionAgent is able to be
initialized with spanner instances (instead of only component instances).