Remove approximation in ransac circle model

[sciunto]

Description

closes #5186

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[pep8speaks]

Hello @sciunto! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file skimage/measure/fit.py:

Line 289:28: E226 missing whitespace around arithmetic operator
Line 289:40: E226 missing whitespace around arithmetic operator
Line 299:28: E226 missing whitespace around arithmetic operator
Line 299:40: E226 missing whitespace around arithmetic operator
Line 300:34: E226 missing whitespace around arithmetic operator
Line 301:34: E226 missing whitespace around arithmetic operator
Line 311:27: E226 missing whitespace around arithmetic operator
Line 311:42: E226 missing whitespace around arithmetic operator