Remove unnecessary Hardhat-related CI jobs

[fvictorio]

As discussed in #752, this removes two Hardhat-related jobs that are not necessary and that will stop working soon when we remove those tests from the Hardhat repo.

[coveralls]

coverage: 84.537%. remained the same
when pulling 42d8696 on fvictorio:remove-unnecessary-ci-jobs
into 4aec1e7 on ethereum:master.

[fvictorio]

The second commit fixes an unrelated thing in the CI: it assumes that the Hardhat repo only has releases related to Hardhat, but we also publish our plugins releases there. This means that if the latest release is a plugin, the job fails.

I fixed that by getting all releases, and getting the first one whose tag starts with hardhat@.

The node-v10 job is still failing, but that also seems unrelated to my changes here.

[r0qs]

Thank you very much @fvictorio! It looks good to me, except for one small comment :)

[r0qs]

The node-v10 job is still failing, but that also seems unrelated to my changes here.

Yeah, it is unrelated. I will fix it in a follow up PR, thanks.

[r0qs]

@cameel this cannot be merged because hardhat-core-latest-solc is marked as required and I don't have access to the repo settings to change that. Note that this job was removed by this PR ;)

[fvictorio]

That job must be failing because I removed those tests from the Hardhat repo thinking this PR was already merged, sorry about that!

[cameel]

Required jobs updated.