Remove unreferenced footnote

Change-Id: I6988ef4f0c6a98c7cbf44a45c3c11c81ab26466e

docs/source/nnls_tutorial.rst

diff --git a/docs/source/nnls_tutorial.rst b/docs/source/nnls_tutorial.rst
index 6de800e..2304e70 100644
--- a/docs/source/nnls_tutorial.rst
+++ b/docs/source/nnls_tutorial.rst
@@ -443,11 +443,6 @@
 :math:`0`. In 10 iterations, Ceres finds a solution with an objective
 function value of :math:`4\times 10^{-12}`.
 
-.. rubric:: Footnotes
-
-.. [#f5] `examples/powell.cc
-   <https://ceres-solver.googlesource.com/ceres-solver/+/master/examples/powell.cc>`_.
-
 
 .. _section-fitting: