[SPARK-8575][SQL] Deprecate callUDF in favor of udf

[BenFradet]

Follow up of SPARK-8356 and #6902.
Removes the unit test for the now deprecated callUdf
Unit test in SQLQuerySuite now uses udf instead of callUDF
Replaced callUDF by udf where possible in mllib

[SparkQA]

Test build #35711 has finished for PR 6993 at commit fe2a10b.

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

[BenFradet]

Pinging @marmbrus

[marmbrus]

/cc @mengxr

[marmbrus]

Style nit: I think we call udf above so the invocation just looks like init().

[BenFradet]

Thanks a lot for your input @mengxr, will update the pr accordingly.

[SparkQA]

Test build #35808 has finished for PR 6993 at commit 0ebd0da.

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

[SparkQA]

Test build #35865 has finished for PR 6993 at commit 1ddb452.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #35867 has finished for PR 6993 at commit 26f5a7a.

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

[BenFradet]

pinging @mengxr and @marmbrus, there is still one occurrence left of callUDF in Transformer but I don't think we can replace this one by udf, can we?

[mengxr]

@BenFradet I don't think it is easy to replace that one because of outputDataType. The changes LGTM. Merged into master. Thanks!