Fix CI linting

[sgsutcliffe]

Fixed the nf-core lint used for CI. Fixed the "tests failed", namely the removed the defaults for follow parameters in 'nextflow_schema.json' file

• params.pointfinder_database
• params.plasmidfinder_database
• params.mlst_scheme

PR checklist

• [ ] This comment contains a description of changes (with reason).
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [ ] If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• [ ] If necessary, also make a PR on the nf-core/staramr branch on the nf-core/test-datasets repository.
• [ ] Make sure your code lints (nf-core lint).
• [ ] Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• [ ] Usage Documentation in docs/usage.md is updated.
• [ ] Output Documentation in docs/output.md is updated.
• [ ] CHANGELOG.md is updated.
• [ ] README.md is updated (including new tool citations and authors/contributors).

[sgsutcliffe]

@nf-core-bot fix linting