Uninstall unregistered extensions

[larohra]

Description

Currently the Agent doesn't uninstall extensions if the extension was unregistered from PIR. This PR handles that scenario - Issue #1449
Additionally did some code cleanup in the PR.

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PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• Except for special cases involving multiple contributors, the PR is started from a fork of the main repository, not a branch.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made and Travis.CI is passing.

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.

[codecov]

Codecov Report

Merging #1970 into develop will increase coverage by 0.04%.
The diff coverage is 80.00%.

@@             Coverage Diff             @@
##           develop    #1970      +/-   ##
===========================================
+ Coverage    69.63%   69.67%   +0.04%     
===========================================
  Files           85       85              
  Lines        12024    12028       +4     
  Branches      1680     1680              
===========================================
+ Hits          8373     8381       +8     
+ Misses        3275     3273       -2     
+ Partials       376      374       -2     

Impacted Files	Coverage Δ
azurelinuxagent/ga/exthandlers.py	86.74% <80.00%> (+0.44%)	⬆️

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