fix: fix usage plan SHARED bug

[keetonian]

Issue #, if available:
N/A
Related to PR #1179
Description of changes:
Usage plans, when the SHARED option was selected, was adding the same api multiple times.
Description of how you validated changes:
make pr and updated tests
Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

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

[codecov-io]

Codecov Report

Merging #1434 into release/v1.21.0 will increase coverage by <.01%.
The diff coverage is 100%.

@@                 Coverage Diff                 @@
##           release/v1.21.0    #1434      +/-   ##
===================================================
+ Coverage            94.37%   94.38%   +<.01%     
===================================================
  Files                   78       78              
  Lines                 4678     4681       +3     
  Branches               934      937       +3     
===================================================
+ Hits                  4415     4418       +3     
  Misses                 121      121              
  Partials               142      142

Impacted Files	Coverage Δ
samtranslator/model/api/api_generator.py	95.07% <100%> (+0.03%)	⬆️

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