k256: remove Scalar::conditional_add_bit

[tarcieri]

The case it's actually being used for in practice is much simpler and can be easily expressed using the current functionality.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.70%. Comparing base (f594fd4) to head (ebea79b).
Report is 644 commits behind head on master.

Files with missing lines	Patch %	Lines
k256/src/arithmetic/scalar/wide32.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
- Coverage   64.74%   64.70%   -0.04%     
==========================================
  Files          28       28              
  Lines        3602     3598       -4     
==========================================
- Hits         2332     2328       -4     
  Misses       1270     1270              

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[fjarri]

Just as a general information: in libsecp256k1, the analogue of this function (secp256k1_scalar_cadd_bit()) is only ever used with a non-zero shift in secp256k1_wnaf_const().