Remove invalid alert_grouping_parameters fields after strict check

[cjgajard]

Fixes #887

$ go test ./pagerduty -v -run TestAccPagerDutyService_
=== RUN   TestAccPagerDutyService_import
--- PASS: TestAccPagerDutyService_import (15.39s)
=== RUN   TestAccPagerDutyService_Basic
--- PASS: TestAccPagerDutyService_Basic (27.50s)
=== RUN   TestAccPagerDutyService_FormatValidation
--- PASS: TestAccPagerDutyService_FormatValidation (46.18s)
=== RUN   TestAccPagerDutyService_AlertGrouping
--- PASS: TestAccPagerDutyService_AlertGrouping (23.35s)
=== RUN   TestAccPagerDutyService_AlertContentGrouping
--- PASS: TestAccPagerDutyService_AlertContentGrouping (70.74s)
=== RUN   TestAccPagerDutyService_AlertContentGroupingIntelligentTimeWindow
--- PASS: TestAccPagerDutyService_AlertContentGroupingIntelligentTimeWindow (21.59s)
=== RUN   TestAccPagerDutyService_AutoPauseNotificationsParameters
--- PASS: TestAccPagerDutyService_AutoPauseNotificationsParameters (27.41s)
=== RUN   TestAccPagerDutyService_BasicWithIncidentUrgencyRules
--- PASS: TestAccPagerDutyService_BasicWithIncidentUrgencyRules (32.74s)
=== RUN   TestAccPagerDutyService_FromBasicToCustomIncidentUrgencyRules
--- PASS: TestAccPagerDutyService_FromBasicToCustomIncidentUrgencyRules (20.89s)
=== RUN   TestAccPagerDutyService_SupportHoursChange
--- PASS: TestAccPagerDutyService_SupportHoursChange (21.23s)
=== RUN   TestAccPagerDutyService_ResponsePlay
--- PASS: TestAccPagerDutyService_ResponsePlay (31.76s)
=== RUN   TestAccPagerDutyService_AlertGroupingParametersAddConfigField
--- PASS: TestAccPagerDutyService_AlertGroupingParametersAddConfigField (21.51s)
PASS
ok  	github.com/PagerDuty/terraform-provider-pagerduty/pagerduty	360.697s