Add clean_decontam target

[Ulthran]

• I have run pytest .tests/ on a local deployment and the tests passed successfully
• If this adds a new output file, I have added a check to .tests/targets.txt
• If this fixes a bug, I have added or modified an appropriate test
• If this adds or modifies a rule that uses FASTQ files, the input accepts gzipped FASTQ and outputs gzipped FASTQ, or marks uncompressed FASTQ output as temp

If this is for a release:

• I have updated documentation
• I have updated the hardcoded version at the top of install.sh to match what this release's version will be
• I have created a release archive that will be attached to this release