Remove signing for files and MSI packages

[dliappis]

After #212 has been merged, the signing will be taken care of by the unified release, so we don't need signing capabilities in this framework anymore.

Practically it would mean removing:

• https://github.com/elastic/elastic-stack-installers/blob/main/src/build/BullseyeTargets/SignMsiPackageTarget.cs
• https://github.com/elastic/elastic-stack-installers/blob/main/src/build/BullseyeTargets/SignProductBinariesTarget.cs

and references to them.

[dliappis]

fyi @jlind23 / @alpar-t

[amitkanfer]

@dliappis should we close this issue?

[jlind23]

The files mentioned above haven't been cleaned up yet so I don't think we are ready to close this one out.