[SPARK-22852][BUILD] Exclude -Xlint:unchecked from sbt javadoc flags

[easel]

What changes were proposed in this pull request?

Moves the -Xlint:unchecked flag in the sbt build configuration from Compile to (Compile, compile) scope, allowing publish and publishLocal commands to work.

How was this patch tested?

Successfully published the spark-launcher subproject from within sbt successfully, where it fails without this patch.

[vanzin]

ok to test

[SparkQA]

Test build #85213 has finished for PR 20040 at commit 1827c2d.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #4020 has finished for PR 20040 at commit 1827c2d.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[SparkQA]

Test build #85257 has finished for PR 20040 at commit 1827c2d.

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

[srowen]

Merged to master