Revert "Updated libstout."

This reverts commit e4ea43d6a3e2059cf4bea101c74cc78fe3d38cb6.
Revert "Added libstout as a submodule."

This reverts commit 7d0168cd2d9bd1377881b5f0fde68f40c12b2901.
Synchronized with changes made in Mesos.