This patch fixes a build break in maven caused by #6441.

Note that this patch reverts the changes in flume-sink because
this module does not currently depend on Spark core, but the
tests require it. There is not an easy way to make this work
because mvn test dependencies are not transitive (MNG-1378).

For now, we will leave the one test suite in flume-sink out
until we figure out a better solution. This patch is mainly
intended to unbreak the maven build.

Author: Andrew Or <andrew@databricks.com>

Closes #6511 from andrewor14/fix-build-mvn and squashes the following commits:

3d53643 [Andrew Or] [HOT FIX #6441] Fix maven build failures