Gaia is removed from cosmos-sdk repository.

[alessio]

Few changes were required to make sure no packages depend on gaia subpackages.

CI config is amended accordingly.

Unnecessary targets are removed from Makefile.

Simulations run through a lightweight version of gaia renamed to simapp.

Closes: #4104

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: clog add [section] [stanza] [message]
• rereviewed Files changed in the github PR explorer

---

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 #4369 into master will decrease coverage by <.01%.
The diff coverage is 30%.

@@            Coverage Diff             @@
##           master    #4369      +/-   ##
==========================================
- Coverage   58.09%   58.09%   -0.01%     
==========================================
  Files         235      235              
  Lines       14880    14879       -1     
==========================================
- Hits         8645     8644       -1     
  Misses       5607     5607              
  Partials      628      628