Remove plugin, cuda related code in automake & autoconf files (#4789)

* Build plugin example with CMake.

* Remove plugin, cuda related code in automake & autoconf files.

* Fix typo in GPU doc.

tests/travis/run_test.sh

@@ -5,8 +5,6 @@ make -f dmlc-core/scripts/packages.mk lz4
 
 if [ ${TRAVIS_OS_NAME} == "osx" ]; then
     echo 'USE_OPENMP=0' >> config.mk
-    echo 'TMPVAR := $(XGB_PLUGINS)' >> config.mk
-    echo 'XGB_PLUGINS = $(filter-out plugin/lz4/plugin.mk, $(TMPVAR))' >> config.mk
 else
     # use g++-4.8 for linux
     export CXX=g++-4.8