modify template to use nf-validation plugin

[mirpedrol]

Closes #1543

The validation of nextflow_schema.json files from nf-core pipelines is now part of a core Nextflow plugin: https://github.com/nextflow-io/nf-validation/pull/5

Functions can be imported to a pipeline with include { paramsHelp, paramsSummaryMap, paramsSummaryLog } from 'plugin/nf-validation'

Schema validation code from the pipeline template NfcoreSchema.groovy and MainWorkflow.groovy is removed.

PR checklist

• [ ] This comment contains a description of changes (with reason)
• [ ] CHANGELOG.md is updated
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [ ] Documentation in docs is updated