ci: remove mergeable configuration

Mergeable is disabled because it was requiring
that PR title follows conventional commit notation
even when PR consisted of multiple commits
and was not planned to be squash merged.

.github/mergeable.yml

@@ -1,15 +0,0 @@
-version: 2
-mergeable:
-  - when: pull_request.*
-    name: "Conventional Commits"
-    validate:
-      - do: title
-        begins_with:
-          match: ['feat', 'fix', 'api', 'refactor', 'perf', 'test', 'style', 'chore', 'cargo', 'build', 'ci', 'docs']
-
-    fail:
-      - do: checks
-        status: "action_required"
-        payload:
-          title: PR title should follow conventional commits
-          summary: "PR title should follow https://conventionalcommits.org. See https://github.com/deltachat/deltachat-core-rust/blob/master/CONTRIBUTING.md for details."