Temporarily do not run system tests

[allileong]

What does this PR do?

Disable system tests until tests are updated

What issues does this PR fix or reference?

[lcampos]

I think we'll also need to remove the usage of npm run coverage:system-test from .appveyor.yml

[codecov]

Codecov Report

Merging #559 into develop will decrease coverage by 0.56%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #559      +/-   ##
===========================================
- Coverage    73.91%   73.35%   -0.57%     
===========================================
  Files          149      149              
  Lines         5832     5832              
  Branches       917      917              
===========================================
- Hits          4311     4278      -33     
- Misses        1289     1328      +39     
+ Partials       232      226       -6

Impacted Files	Coverage Δ
...kages/salesforcedx-vscode-apex/src/requirements.ts	27.5% <0%> (-35%)	⬇️
...-generator/test/integration/integrationTestUtil.ts	21.21% <0%> (-19.7%)	⬇️
...ges/salesforcedx-vscode-apex/src/languageServer.ts	40.74% <0%> (-11.12%)	⬇️

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