mgm: remove unnecessary NewBlockCipher bounds

[newpavlov]

This change allows us to work with Mgm<&'a BlockCipher>.

[codecov-commenter]

Codecov Report

Merging #314 (1da77fe) into master (99bf36c) will not change coverage.
The diff coverage is n/a.

❗ Current head 1da77fe differs from pull request most recent head ddf4b78. Consider uploading reports for the commit ddf4b78 to get more accurate results

@@           Coverage Diff           @@
##           master     #314   +/-   ##
=======================================
  Coverage   84.47%   84.47%           
=======================================
  Files          31       31           
  Lines        1140     1140           
=======================================
  Hits          963      963           
  Misses        177      177           

Impacted Files	Coverage Δ
mgm/src/lib.rs	96.55% <ø> (ø)

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