Rft: remove defaultRegistry

[flycash]

Users should make the decision that if use "global regitry". If they want, they should define their own global registry.

[codecov]

Codecov Report

Merging #46 (2d407fd) into main (3373867) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   79.36%   79.61%   +0.24%     
==========================================
  Files          15       14       -1     
  Lines         727      721       -6     
==========================================
- Hits          577      574       -3     
+ Misses        133      129       -4     
- Partials       17       18       +1     

Impacted Files	Coverage Δ
model.go	84.21% <ø> (-7.90%)	⬇️
db.go	84.41% <100.00%> (ø)

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