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]

Codecov Report

Merging #431 (ebea79b) into master (f594fd4) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            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              

Impacted Files	Coverage Δ
k256/src/arithmetic/scalar.rs	84.13% <ø> (-0.22%)	⬇️
k256/src/arithmetic/scalar/wide32.rs	0.00% <0.00%> (ø)
k256/src/arithmetic/scalar/wide64.rs	95.78% <100.00%> (ø)

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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().