docs: Removed unused Doxyfile

docs: Removed unused Doxyfile

There are two Doxyfiles for generating C API documentation
1. ./resource/docs/c-doc/Doxyfile
2. ./resource/csdk/doc/Doxyfile

The first file is listed in the wiki and appears to be
better maintained than the second. To avoid confusion
the second is removed by this commit.

Change-Id: I5fe1f688bbd9930de623fad26494a880bac35244
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17909
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>