fix: revert adding GlobalClusterEndpoint to readonlyProperty (#1631)

In a recent schema change, globalEndpoint property was added to
readOnlyPorperties
```
"readOnlyProperties" : [ "/properties/GlobalEndpoint" ],
```

This caused `GlobalEndpoint` property is being removed from RDS
GlobalCluster L1.

Reverting this change for now to continue the release.

Author: xazhao