Remove processes/ReleaseProcess (replaced by gitlab issue template)

processes/ReleaseProcess needs some updates after last signing process changes. However, since everything is already documented in tor-browser-build/.gitlab/issue_templates/Release Prep - Tor Browser Stable.md (and Release Prep - Tor Browser Stable.md for alpha), I think we should check if something from processes/ReleaseProcess is missing in the gitlab issue template, and then remove processes/ReleaseProcess.

/cc @richard

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