buildlet: stop explicitly selecting random availability zones on EC2

This change stops explicitly selecting a random availability zone upon
EC2 vm creation if one is not set. The EC2 api will select an
availability zone with capacity for the request when an availability
zone is not set. The caller maintains the ability to set an
availability zone if needed.

Updates golang/go#36841

Change-Id: I87d4e8e66fcfadfedbe584a2c016696dc4d62062
Reviewed-on: https://go-review.googlesource.com/c/build/+/247901
Run-TryBot: Carlos Amedee <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: cagedmantis

buildlet/ec2.go

@@ -58,9 +58,6 @@ func (c *EC2Client) StartNewVM(ctx context.Context, buildEnv *buildenv.Environme
 	if opts.Description == "" {
 		opts.Description = fmt.Sprintf("Go Builder for %s", hostType)
 	}
-	if opts.Zone == "" {
-		opts.Zone = buildEnv.RandomEC2VMZone()
-	}
 	if opts.DeleteIn == 0 {
 		opts.DeleteIn = 30 * time.Minute
 	}