plugins: remove DriverNetwork.Hash method

[tgross]

As part of a project to make FIPS-140 compliant binaries available for Nomad Enterprise customers, we intend to use the Go Cryptographic Module. In GODEBUG=fips140=only mode ("enforcing"), the md5 hash function is disallowed and will panic the agent if you try to call md5.New().

The DriverNetwork object is used only by the docker driver and we never use the Hash function in Nomad. The method was originally introduced in v0.6.0, (ref 4117c8b) where it was used as part of an optimization meant to reduce writes to the client state database. When the client was reworked in 0.9 it was moved into the plugins package, where it no longer had a purpose.

Unfortunately this method is exposed to external drivers that import the plugins/drivers package even though this isn't a tagged released package intended for import. (Including our own external drivers.) I've reviewed all known community drivers for use of this method, and none of them use it. I considered switching this to a SHA256 hash, but that would silently break private plugins we don't know about, as opposed to removing the Hash() method and breaking them at compile time, which should be an easily fixed situation. Another alterative would be to leave the method in place because it's not going to be called inside our binary and therefore safe for FIPS builds, but that's just asking for trouble by having someone use it later.

Remove the unused method.

Ref: https://hashicorp.atlassian.net/browse/NMD-1658
Ref: https://go.dev/doc/security/fips140
Ref: https://go.dev/blog/fips140

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Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[jrasell]

LGTM!