Clearing Y.1731 statistics

Use the clear cfm y1731 statistics command to clear statistics for all Y1731 entities.
```
device# clear cfm y1731 statistics
```
Use the clear cfm y1731 statistics delay-mesurement command to clear all statistics for Two-Way ETH-DM.
```
device# clear cfm y1731 statistics delay-mesurement
```
Use the clear cfm y1731 statistics synthetic-loss-measurement command to clear all statistics for Two-Way ETH-SLM.
```
device# clear cfm y1731 statistics synthetic-loss-measurement
```