Remove Util.getJson(String)

This method was used only in a test. Remove it, and replace its
uses in the test by JsonParser.parseString() directly. Since the
test strings are all known to be objects, casting to JsonObject
is fine.

Change-Id: If2c05cea20ec03f188fc76f143f5f66bbb76e133
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>