We currently have support for generating tags via ctags as part of our
build system. We aren't really in the place of supporting any tooling
that exists apart from the actual build environment, as doing so adds
additional complexity and maintenance burden to our build instructions.
This is in fact nicely demonstrated by this particular option, as it
hasn't been working anymore since commit e5c9723d (cmake: move library
build instructions into subdirectory, 2017-06-30).
As a result, this commit removes support for building CTags
