Revert "Feature: Allow getting credentials via EC2 role (#3343)" (#3985)

This reverts commit 808f79f00a.

discovery/ec2/ec2.go

@@ -22,7 +22,6 @@ import (
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/credentials"
-	"github.com/aws/aws-sdk-go/aws/credentials/ec2rolecreds"
 	"github.com/aws/aws-sdk-go/aws/credentials/stscreds"
 	"github.com/aws/aws-sdk-go/aws/ec2metadata"
 	"github.com/aws/aws-sdk-go/aws/session"
@@ -186,16 +185,6 @@ func (d *Discovery) Run(ctx context.Context, ch chan<- []*targetgroup.Group) {
 	}
 }
 
-func (d *Discovery) ec2MetadataAvailable(sess *session.Session) (isAvailable bool) {
-	svc := ec2metadata.New(sess, &aws.Config{
-		MaxRetries: aws.Int(0),
-	})
-
-	isAvailable = svc.Available()
-
-	return isAvailable
-}
-
 func (d *Discovery) refresh() (tg *targetgroup.Group, err error) {
 	t0 := time.Now()
 	defer func() {
@@ -218,12 +207,7 @@ func (d *Discovery) refresh() (tg *targetgroup.Group, err error) {
 		creds := stscreds.NewCredentials(sess, d.roleARN)
 		ec2s = ec2.New(sess, &aws.Config{Credentials: creds})
 	} else {
-		if d.aws.Credentials == nil && d.ec2MetadataAvailable(sess) {
-			creds := ec2rolecreds.NewCredentials(sess)
-			ec2s = ec2.New(sess, &aws.Config{Credentials: creds})
-		} else {
-			ec2s = ec2.New(sess)
-		}
+		ec2s = ec2.New(sess)
 	}
 	tg = &targetgroup.Group{
 		Source: *d.aws.Region,