Retrieve OpenVINO sources during build.rs instead of as a Git submodule

[abrown]

When building openvino-rs, if there is no installed version of OpenVINO available, Cargo attempts to build OpenVINO itself using the sources pulled in as a Git submodule. The sources do not fit in to the 10MB crate limit, so this build option will not work for users of openvino-rs (hence the addition of OPENVINO_SKIP_LINKING to allow, e.g., docs.rs to build the crate).

Instead, we could add git2 as a dev dependency and pull the sources from there to build. This is not optimal for several reasons: it is slow and it alters the sources during a Cargo build (an anti-pattern). There are examples of this type of thing being done, though; intel-mkl-src is pulling binaries from S3.

Alternately, the "build from source" should just be removed.