remove duplicate section in usage.md

[fbnrst]

The Samplesheet input section was duplicated in usage.md, I removed one of them. As this is just and update on the documentation I did not go through the full PR checklist

PR checklist

• This comment contains a description of changes (with reason).
• Usage Documentation in docs/usage.md is updated.

[github-actions]

This PR is against the master branch ❌

• Do not close this PR
• Click Edit and change the base to dev
• This CI test will remain failed until you push a new commit

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Hi @fbnrst,

It looks like this pull-request is has been made against the fbnrst/methylseq master branch.
The master branch on nf-core repositories should always contain code from the latest release.
Because of this, PRs to master are only allowed if they come from the fbnrst/methylseq dev branch.

You do not need to close this PR, you can change the target branch to dev by clicking the "Edit" button at the top of this page.
Note that even after this, the test will continue to show as failing until you push a new commit.

Thanks again for your contribution!