k256/p256: remove Generate trait

[tarcieri]

It's been replaced by methods like Field::random and Group::random.

See: RustCrypto/traits#295

[codecov-commenter]

Codecov Report

Merging #175 into master will increase coverage by 0.07%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
+ Coverage   58.21%   58.28%   +0.07%     
==========================================
  Files          25       25              
  Lines        3786     3778       -8     
==========================================
- Hits         2204     2202       -2     
+ Misses       1582     1576       -6     

Impacted Files	Coverage Δ
k256/src/ecdsa.rs	85.71% <ø> (ø)
k256/src/ecdsa/recoverable.rs	62.33% <ø> (ø)
k256/src/ecdsa/sign.rs	30.90% <0.00%> (ø)
p256/src/arithmetic/scalar.rs	77.41% <ø> (+0.29%)	⬆️
p256/src/ecdsa.rs	90.47% <ø> (ø)
k256/src/arithmetic.rs	100.00% <100.00%> (ø)
k256/src/arithmetic/scalar.rs	76.02% <100.00%> (+1.12%)	⬆️
p256/src/arithmetic.rs	100.00% <100.00%> (ø)
p256/src/arithmetic/scalar/blinding.rs	91.66% <100.00%> (ø)
... and 1 more

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