Large parts the library weren't being used, and the parts that were was overly
abstracted for our use case. Additionally, part of its use case was to abstract
pkgconfig boilerplate, which is no longer needed as pkgconfig has been removed
in favor of relying on cmake alone in 09118052ce.
The variables are not meant to be defined there really, but only with
the third-party project.
Using them, e.g. with the following, would actually result in libvterm
not being found then:
make CMAKE_EXTRA_FLAGS="-DUSE_BUNDLED_LIBVTERM=ON" \
DEPS_CMAKE_FLAGS="-DUSE_BUNDLED=OFF -DUSE_BUNDLED_LIBVTERM=ON"
In https://github.com/neovim/neovim/pull/6357 they were renamed to
`USE_BUNDLED_X` from `X_USE_BUNDLED`, but the above reasoning applies
to the old names, too.
Internally `CMAKE_PREFIX_PATH` is used to add the built/bundled third
party packages for `find_package`, so there is no reason to e.g. query
the values via `load_cache` for example from the third-party project.
