Remove varianble's 'def' from samtool sort 'output_file'

[bagnacan]

This PR removes a variable's def to fix an error encountered while running samtools/sort using Nextflow 24.04.2:

ERROR ~ Module compilation error
- file : /path/to/pipeline/./workflows/../modules/nf-core/samtools/sort/main.nf
- cause: Variable `prefix` already defined in the process scope @ line 36, column 26.
       def output_file = "${prefix}.${extension}"

PR checklist

• This comment contains a description of changes (with reason).
• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
  • For modules:
    • nf-core modules test <MODULE> --profile docker
    • nf-core modules test <MODULE> --profile singularity
    • nf-core modules test <MODULE> --profile conda