Simplify single task retry strategy check

[eapolinario]

Why are the changes needed?

Make the intent of single task retry check more clear

What changes were proposed in this pull request?

Remove unnecessary checks. Since core.RetryStrategy is a protobuf message we assume that calls to Get* are guarded against nil.

How was this patch tested?

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.91%. Comparing base (a07753e) to head (f122672).
Report is 117 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5584   +/-   ##
=======================================
  Coverage   35.90%   35.91%           
=======================================
  Files        1301     1301           
  Lines      109403   109401    -2     
=======================================
+ Hits        39282    39286    +4     
+ Misses      66024    66018    -6     
  Partials     4097     4097           

Flag	Coverage Δ
unittests-datacatalog	51.37% <ø> (ø)
unittests-flyteadmin	53.73% <100.00%> (+0.02%)	⬆️
unittests-flytecopilot	12.17% <ø> (ø)
unittests-flytectl	62.28% <ø> (ø)
unittests-flyteidl	7.09% <ø> (ø)
unittests-flyteplugins	53.31% <ø> (ø)
unittests-flytepropeller	41.75% <ø> (ø)
unittests-flytestdlib	55.25% <ø> (ø)

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[katrogan]

thank you :)