remove color label in Nextflow image from readme linting

[mirpedrol]

Close #1589

Remove color label in Nextflow image from readme linting to fix "readme: README did not have a Nextflow minimum version badge." warning.

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

[codecov]

Codecov Report

Merging #1590 (8aaccfd) into dev (c560450) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1590      +/-   ##
==========================================
+ Coverage   64.58%   64.63%   +0.04%     
==========================================
  Files          54       54              
  Lines        6249     6249              
==========================================
+ Hits         4036     4039       +3     
+ Misses       2213     2210       -3     

Impacted Files	Coverage Δ
nf_core/lint/readme.py	79.31% <100.00%> (+10.34%)	⬆️

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[drpatelh]

Thanks @mirpedrol !