chore: remove obsolete FIXME comments from bipartite tests

[dwaddle]

Describe your change:

Removed 4 obsolete FIXME comments from graphs/check_bipatrite.py that were left behind
in a previous PR (#10708). The bugs they referenced (KeyError and TypeError) were
already fixed at that time by migrating to a defaultdict and adding node existence checks,
making these FIXME comments and tracebacks redundant. No functional code or tests were
changed.

• Add an algorithm?
• [ X] Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• [ X] Documentation change?

Checklist:

• [ X] I have read CONTRIBUTING.md.
• [ X] This pull request is all my own work -- I have not plagiarized.
• [ X] I know that pull requests will not be merged if they fail the automated tests.
• [ X] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ X] All new Python files are placed inside an existing directory.
• [ X] All filenames are in all lowercase characters with no spaces or dashes.
• [ X] All functions and variable names follow Python naming conventions.
• [ X] All function parameters and return values are annotated with Python type hints.
• [ X] All functions have doctests that pass the automated testing.
• [ X] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[algorithms-keeper]

Closing this pull request as invalid

@dwaddle, this pull request is being closed as none of the checkboxes have been marked. It is important that you go through the checklist and mark the ones relevant to this pull request. Please read the Contributing guidelines.

If you're facing any problem on how to mark a checkbox, please read the following instructions:

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• If it is, then mark it by putting a x between the square bracket like so: [x]

NOTE: Only [x] is supported so if you have put any other letter or symbol between the brackets, that will be marked as invalid. If that is the case then please open a new pull request with the appropriate changes.