baseapp: Remove baseapp.SetTxDecoder()

[ValarDragon]

Closes #1441

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Updated relevant documentation (docs/) - nothing showed up in documentation with ag settxdecoder.

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #2230 into develop will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2230      +/-   ##
===========================================
+ Coverage     64.2%   64.23%   +0.02%     
===========================================
  Files          140      140              
  Lines         8579     8575       -4     
===========================================
  Hits          5508     5508              
+ Misses        2694     2690       -4     
  Partials       377      377

[cwgoes]

utACK

The best kind of PR!