Remove viper global single from x/staking

[jgimeno]

Description

closes: #XXXX

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• 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.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #6631 into master will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6631      +/-   ##
==========================================
- Coverage   57.58%   57.50%   -0.09%     
==========================================
  Files         500      500              
  Lines       29999    30024      +25     
==========================================
- Hits        17275    17264      -11     
- Misses      11479    11515      +36     
  Partials     1245     1245              

[lgtm-com]

This pull request introduces 1 alert when merging a7d4a58 into a459b78 - view on LGTM.com

new alerts:

• 1 for Useless assignment to local variable

[alexanderbez]

ACK