[SPARK-4397][Core] Cleanup 'import SparkContext._' in core

[zsxwing]

This PR cleans up import SparkContext._ in core for SPARK-4397(#3262) to prove it really works well.

[SparkQA]

Test build #23977 has started for PR 3530 at commit 04e2273.

• This patch merges cleanly.

[JoshRosen]

This is a really important fix, actually, since we ran into problems with IntelliJ's automatic import cleanup removing these: if we perform this import cleanup incrementally as part of other patches, then those patches will introduce build-breaks if they're cherry-picked into pre-1.2 versions of Spark. As a result, it's much better to do all of this cleanup in one pass, as you've done here.

+1.

[SparkQA]

Test build #23977 has finished for PR 3530 at commit 04e2273.

• 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/23977/
Test PASSed.

[rxin]

Thanks. Merging in master.

[zzcclp]

Whether this PR will be merged into the branch 1.2?

[zsxwing]

No. This is for SPARK-4397(#3262) and #3262 wasn't merged to branch 1.2.