[CORE][minor] remove unnecessary ClassTag in DAGScheduler

[cloud-fan]

This existed at the very beginning, but became unnecessary after this commit. I think we should remove it if we don't plan to use it in the future.

[AmplabJenkins]

Can one of the admins verify this patch?

[rxin]

Jenkins, test this please.

[SparkQA]

Test build #28501 has started for PR 4992 at commit e857f2e.

• This patch merges cleanly.

[SparkQA]

Test build #28501 has finished for PR 4992 at commit e857f2e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28501/
Test PASSed.

[srowen]

LGTM