DebugInfo: Remove the empty macinfo section.

We were just emitting a label for this section for no real reason - this
caused us to emit the section even though we never put anything in it.

Not bothering with a test (though not adamantly anti-test) because it
seems somewhat arbitrary to test for the absence of this section anymore
than the absence of any other section.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201876 91177308-0d34-0410-b5e6-96231b3b80d8