Remove self from disconnect() method when unload is called

[FabioRosado]

Description

I was a bit excited with merging #749 into opsdroid. I was updating the connectors and see if the shell connector was working as expected with the changes and noticed that opsdroid would hang on ctrl+c because the unload method was calling disconnect on connectors and db with self as a param.

This PR fixes those two disconnect methods and makes it possible to terminate opsdroid properly.

Fixes #

Status

READY | UNDER DEVELOPMENT | ON HOLD

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

• Tox - all green
• Started opsdroid, killed it, exited successfully.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #775 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #775   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          28     28           
  Lines        1850   1850           
=====================================
  Hits         1850   1850

Impacted Files	Coverage Δ
opsdroid/core.py	100% <100%> (ø)	⬆️

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