Reduce OptionT from MonadCombine to Monad

[mikejcurry]

Reduces OptionT MonadCombine instance to Monad.

The MonadCombine instance for OptionT was not tested and does not pass laws.

This addresses #693.
#694 exists for tracking potential changes to laws which may allow this instance, or an alternative type class instance, to be reinstated.

[codecov-io]

Current coverage is 84.23%

Merging #698 into master will increase coverage by +0.08% as of 1d3fed4

@@            master    #698   diff @@
======================================
  Files          162     162       
  Stmts         2222    2220     -2
  Branches        74      74       
  Methods          0       0       
======================================
  Hit           1870    1870       
  Partial          0       0       
+ Missed         352     350     -2

Review entire Coverage Diff as of 1d3fed4

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[ceedubs]

👍 thanks!

[adelbertc]

👍