Remove `WORKSPACE` writing functionality.

We perform a write of canonical arguments to WORKSPACE when performing a download.

This is a terrible practice. The metadata attribute added in !10 (merged) is a much better way to control this.

Remove the //lib:workspace.bzl.

Any return write(rctx, canonical) can be switched to return canonical.

We could have a graceful way to resolve this using environment variables:

• Do not do any WORKSPACE writing if BAZEL_DOWNLOAD_UTILS_WRITE_WORKSPACE=0
• Possibly, print a single print warning about the deprecation
• Do a release and make it explicit that the functionality is deprecated and will eventually be removed
• Wait a few months
• Flip the functionality so that we only write the WORKSPACE file when BAZEL_DOWNLOAD_UTILS_WRITE_WORKSPACE=1
• Wait a few months
• Remove the functionality