Remove PlaygroundQuickLook test.

Historically, this test was verifying some reflection support that was
used for Playgrounds support. Today, through API changes, this was just
testing that we could create an print a specific enum.

With the switch to Swift 5.0 (swiftlang/swift#20374),
this enum is now deprecated and the test fails.

As the coverage provided here seems pretty insignificant, let's just remove
the test.

Author: fredriss