Remove unnecessary #include

src/test-gradient-horrible-logarithmic.cc

src/test-gradient-horrible.cc

src/test-gradient-identity.cc

src/test-gradient-sample-3d.cc

src/test-gradient-sample-nonsmooth.cc

src/test-gradient-sample-steep.cc

src/test-gradient-sample-steep2.cc

src/test-gradient-sample-verysteep.cc

src/test-gradient-sample.cc

src/test-gradient-sample2.cc

src/test-gradient-trivial.cc