[BUGFIX] - Reverting readOnly for two fields

* In this already-merged PR
(Azure#1429), Ben made some
object-refactoring changes - one of them being to make all the fields in
USqlJobProperties read-only as a good design decision.  However, by
making the fields statistics and debugData read-only, this caused some
conflicts in the Powershell code.  We need to pan out the design more
carefully for these two fields with respect to their associated APIs
("/Jobs/{jobIdentity}/GetDebugDataPath" for the debugData field and
"/Jobs/{jobIdentity}/GetStatistics" for the statistics field) before
making them read-only.  If I do not make the change proposed here, I
would have to create two new cmdlets: one for getting the JobDataPath
object and another for getting the JobStatistics object, which we do not
want to do until we have a better understanding of their designs as a
whole.  Hence, the reverting change proposed here.

specification/datalake-analytics/data-plane/Microsoft.DataLakeAnalytics/2016-11-01/job.json

@@ -747,12 +747,10 @@
         },
         "statistics": {
           "$ref": "#/definitions/JobStatistics",
-          "readOnly": true,
           "description": "the job specific statistics."
         },
         "debugData": {
           "$ref": "#/definitions/JobDataPath",
-          "readOnly": true,
           "description": "the job specific debug data locations."
         },
         "diagnostics": {