Remove or comment out zebra internal miner

[oxarbitrage]

In #8136 we added an internal miner to zebra that depends on changes made to the ECC equihash crates. The problem is that this changes are currently in the form of pull requests, unpublished in crates.io. For this reason the release publishing to crates.io of Zebra is failing as all dependencies must be published. More info in #8165 (comment)

We need to fix this in order to complete the current ongoing release (1.5.1) which was made in github but was not published to crates.io.

We decided to remove the internal miner code as a temporal solution until the needed PRs (zcash/librustzcash#1088 and zcash/librustzcash#1083) are merged and a new version is published.

This ticket is to:

• remove the dependency and related code that depends on it.
• open a ticket to restore once stuff gets merged and new version published.

Follow up work

Create a hotfix release (1.5.2) with the changes done.