crypto/keys: remove DecodeSignature from baseKeybase

[alessio]

Keybase implementations should return errors when
signature generation is attempted with offline/multisig
keys since theyr lack private keys.

This is a change in Keyring/Keybase.Sign() methods
semantics that have been broken for long time.

---

For contributor use:

• 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

---

For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5844 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5844      +/-   ##
==========================================
- Coverage   32.33%   32.32%   -0.01%     
==========================================
  Files         353      353              
  Lines       39479    39465      -14     
==========================================
- Hits        12766    12759       -7     
+ Misses      25439    25434       -5     
+ Partials     1274     1272       -2

Impacted Files	Coverage Δ
crypto/keys/keybase_base.go	75.21% <ø> (+2.69%)	⬆️
crypto/keys/keybase.go	65.95% <100%> (ø)	⬆️
crypto/keys/keyring.go	51.91% <100%> (ø)	⬆️

[alexanderbez]

ACK