Remove handling of the unitTestInject* bits in base MTRDevice.

These injections need to be handled by subclasses, because the subclasses do
very different report processing and have different mechanisms for getting
reports.

Once these implementations are removed, the following become unreachable and can
be removed:

* _handleEventReport
* _handleReportBegin
* _handleAttributeReport
* _handleReportEnd

Once those are removed, the following become unreachable or unused and can be
removed:

* _receivingReport (always false)
* _receivingPrimingReport (always false, because _receivingReport is always false)
* _changeState
* _callDelegateDeviceCachePrimed
* unreportedEvents
* _getAttributesToReportWithReportedValues

Once those are removed, the following become unreachable or unused and can be
removed:

* _setCachedAttributeValue
* _noteDataVersion
* _deviceConfigurationChanged
* estimatedStartTimeFromGeneralDiagnosticsUpTime
* _attributeAffectsDeviceConfiguration
* _pruneStoredDataForPath

Once those are removed, the following become unreachable or unused and can be
removed:

* _pruneEndpointsIn
* _pruneClustersIn
* _pruneAttributesIn
* _attributePathAffectsDescriptionData
* _addInformationalAttributesToCurrentMetricScope

Once those are removed, the following become unreachable or unused and can be
removed:

* _removeAttributes
* _removeClusters

Once those are removed, the following become unreachable or unused and can be
removed:

* _removeCachedAttribute

Author: bzbarsky-apple