Remove toolchain Kudu support

IMPALA-7499 is adding support for pulling Kudu in from CDH. For now, to limit impact on developers, we've kept the ability to build with the toolchain Kudu since the CDH Kudu is provided for fewer OSes than the toolchain. In particular the CDH Kudu is not available for Ubuntu 14, which is still used by many devs.

Eventually, we should remove the toolchain Kudu support as it adds complexity to the build system. A good time to do this would be around EOL for Ubuntu 14, currently scheduled for April 2019

Note that even after removing toolchain Kudu support, it will be possible to build and run Impala on OSes for which CDH tarballs are not provided in two ways:

• setting KUDU_IS_SUPPORTED=false, which will build a non-functional Kudu client stub Impala can be linked against
• setting KUDU_CLIENT_DIR/KUDU_BUILD_DIR to point to a local build of Kudu