[SPARK-4904] [SQL] Remove the unnecessary code change in Generic UDF

[chenghao-intel]

Since #3429 has been merged, the bug of wrapping to Writable for HiveGenericUDF is resolved, we can safely remove the foldable checking in HiveGenericUdf.eval, which discussed in #2802.

[SparkQA]

Test build #24645 has started for PR 3745 at commit 622ad03.

• This patch merges cleanly.

[SparkQA]

Test build #24645 has finished for PR 3745 at commit 622ad03.

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

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[marmbrus]

Thanks! Merged to master.