Remove redundant minimal template workspace (#5330)

This removes the workspace of the minimal template, which (I think) is
redundant. The other two templates do not have such a workspace.

The synchronized template created [it's own
workspace](https://github.com/paritytech/polkadot-sdk-minimal-template/blob/master/Cargo.toml)
anyway, and the new readme replaced the old docs contained in `lib.rs`.

Closes
https://github.com/paritytech/polkadot-sdk-minimal-template/issues/11

Silent because the crate was private.