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ARROW-6418: [C++][Plasma] Remove cmake project directive for plasma

Putting plasma into a separate project prevents it from being exported to ArrowTargets.cmake, because targets are added to the export sets based on ${PROJECT_NAME}, but the plasma-targets are neither exported to the build tree nor installed.
An alternative fix for this is to handle the plasma-targets similar to the arrow-targets, but I can't see a benefit for that approach and it would require users to take additional steps.

Closes #5249 from tobim/topic/plasma-targets and squashes the following commits:

19907be73 <Tobias Mayer> Remove cmake project directive for plasma

Authored-by: Tobias Mayer <tobim@fastmail.fm>
Signed-off-by: Wes McKinney <wesm+git@apache.org>