remove an obsolete test

The script was using Data.Set to test handling of unboxed fields by :print,
but some change in Data.Set or in ghc has made fromList more strict and 
the :print pprinter never gets to show the unboxed fields itself.
Moreover, print022 is a much better test for unboxed fields than this one

testsuite/tests/ghc-regress/ghci.debugger/scripts/print015.script

--- Data.Set makes for a wondeful test of unboxed fields
-
-:m +Data.Set
-let s = fromList [1,2,3]
-s
-:p s

testsuite/tests/ghc-regress/ghci.debugger/scripts/print015.stdout

-fromList [1,2,3]
-s = fromList [1,2,3]