ec/suite_b: Remove redundant assertion.

[briansmith]

This eliminates the last should_panic test.

[codecov]

Codecov Report

❌ Patch coverage is 95.52239% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.71%. Comparing base (1bb94b3) to head (dd5e3af).

Files with missing lines	Patch %	Lines
src/ec/suite_b/ecdsa/signing.rs	89.65%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2698   +/-   ##
=======================================
  Coverage   96.70%   96.71%           
=======================================
  Files         187      187           
  Lines       20116    20143   +27     
  Branches      513      512    -1     
=======================================
+ Hits        19454    19481   +27     
- Misses        558      559    +1     
+ Partials      104      103    -1     

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