refactor: Remove ParseApp and tests

[dplewis]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

ParseApp was used in the old parse.com days and no longer works with open source parse server

[parse-github-assistant]

Thanks for opening this pull request!

• ❌ Please link an issue that describes the reason for this pull request, otherwise your pull request will be closed. Make sure to write it as Closes: #123 in the PR description, so I can recognize it.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.59 🎉

Comparison is base (1dace2e) 97.88% compared to head (e4df55e) 98.47%.

❗ Current head e4df55e differs from pull request most recent head a1d343f. Consider uploading reports for the commit a1d343f to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #501      +/-   ##
============================================
+ Coverage     97.88%   98.47%   +0.59%     
+ Complexity     1066     1058       -8     
============================================
  Files            39       38       -1     
  Lines          3262     3213      -49     
============================================
- Hits           3193     3164      -29     
+ Misses           69       49      -20     

Impacted Files	Coverage Δ
src/Parse/ParseCloud.php	100.00% <ø> (ø)
src/Parse/ParseFile.php	100.00% <ø> (ø)
src/Parse/ParsePush.php	100.00% <ø> (ø)
src/Parse/ParseClient.php	95.67% <100.00%> (-0.18%)	⬇️

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[mtrezza]

What was ParseApp? Like an example app?

[dplewis]

parse.dashboard.com/apps you can get the apps listed in your dashboard through the SDK

[parseplatformorg]

🎉 This change has been released in version 2.1.0