llvm-profdata: Don't pipe stderr into show for the tests

Some text shows up on stderr when using guard malloc, and this test
was trying to treat that as input to llvm-profdata show. There's no
reason to pipe stderr into show at all here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204549 91177308-0d34-0410-b5e6-96231b3b80d8