resource/aws_glue_job: Address PR #9115 schema feedback

Reference: #9115 (review)

Output from acceptance testing:

```
--- PASS: TestAccAWSGlueJob_Basic (20.19s)
--- PASS: TestAccAWSGlueJob_Timeout (41.58s)
--- PASS: TestAccAWSGlueJob_Command (51.15s)
--- PASS: TestAccAWSGlueJob_PythonShell (52.94s)
--- PASS: TestAccAWSGlueJob_MaxRetries (57.00s)
--- PASS: TestAccAWSGlueJob_MaxCapacity (58.24s)
--- PASS: TestAccAWSGlueJob_AllocatedCapacity (67.58s)
--- PASS: TestAccAWSGlueJob_GlueVersion (77.25s)
--- PASS: TestAccAWSGlueJob_DefaultArguments (78.53s)
--- PASS: TestAccAWSGlueJob_Description (82.81s)
--- PASS: TestAccAWSGlueJob_ExecutionProperty (90.17s)
--- PASS: TestAccAWSGlueJob_WorkerType (91.13s)
--- PASS: TestAccAWSGlueJob_SecurityConfiguration (105.39s)
```

aws/resource_aws_glue_job.go

@@ -3,7 +3,6 @@ package aws
 import (
 	"fmt"
 	"log"
-	"regexp"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/glue"
@@ -122,13 +121,17 @@ func resourceAwsGlueJob() *schema.Resource {
 			"worker_type": {
 				Type:          schema.TypeString,
 				Optional:      true,
-				ConflictsWith: []string{"allocated_capacity", "max_capacity", "allocated_capacity"},
-				ValidateFunc:  validation.StringMatch(regexp.MustCompile(`^Standard|G.1X|G.2X`), "Must be one of Standard, G.1X or G.2X. See https://docs.aws.amazon.com/glue/latest/dg/aws-glue-api-jobs-job.html"),
+				ConflictsWith: []string{"allocated_capacity", "max_capacity"},
+				ValidateFunc:  validation.StringInSlice([]string{
+					glue.WorkerTypeG1x,
+					glue.WorkerTypeG2x,
+					glue.WorkerTypeStandard,
+				}, false),
 			},
 			"number_of_workers": {
 				Type:          schema.TypeInt,
 				Optional:      true,
-				ConflictsWith: []string{"allocated_capacity", "max_capacity", "allocated_capacity"},
+				ConflictsWith: []string{"allocated_capacity", "max_capacity"},
 				ValidateFunc:  validation.IntAtLeast(2),
 			},
 		},