Remove bundled, unmaintained libmng source code

[ChangeLog][Third-Party Code] The unmaintained bundled libmng has been
removed. Building the mng handler will require libmng to be present as
a system or external library.

Change-Id: I4a4805fa3f3088b5d581b92bfeabba7978bac5f7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>