Remove unnecessary PR-content-check workflow

[popematt]

Issue #, if available:

None

Description of changes:

We can use workflow trigger conditions instead of having a child workflow to check which files have changed.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch coverage: 97.56% and project coverage change: +0.05% 🎉

Comparison is base (6f64aec) 66.83% compared to head (27e1ac9) 66.88%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #541      +/-   ##
============================================
+ Coverage     66.83%   66.88%   +0.05%     
- Complexity     5394     5411      +17     
============================================
  Files           156      156              
  Lines         22741    22769      +28     
  Branches       4082     4095      +13     
============================================
+ Hits          15198    15230      +32     
+ Misses         6247     6243       -4     
  Partials       1296     1296              

Files Changed	Coverage Δ
...rc/com/amazon/ion/impl/bin/IonRawBinaryWriter.java	90.85% <83.33%> (-0.10%)	⬇️
src/com/amazon/ion/impl/bin/WriteBuffer.java	94.95% <100.00%> (+0.31%)	⬆️

... and 1 file with indirect coverage changes

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