Remove unused NEWLINE global

[The-Login]

Removed NEWLINE global variable

What do these changes do?

Removed unused global NEWLINE variable.

Are there changes in behavior for the user?

No

Related issue number

No

Checklist

• I think the code is well written
• Unit tests for the changes exist
• tox testenvs have been executed in the following environments:
  • Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): {py36,py37,py38,py39}-{nocov,cov,diffcov}, qa, docs
  • Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• Documentation reflects the changes
• Add a news fragment into the NEWS.rst file
  • Add under the "aiosmtpd-next" section, creating one if necessary
    • You may create subsections to group the changes, if you like
  • Use full sentences with correct case and punctuation
  • Refer to relevant Issue if applicable

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.84%. Comparing base (9d07336) to head (5a08ed5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #417      +/-   ##
==========================================
- Coverage   97.86%   97.84%   -0.02%     
==========================================
  Files          23       23              
  Lines        5707     5706       -1     
  Branches      764      764              
==========================================
- Hits         5585     5583       -2     
- Misses         77       79       +2     
+ Partials       45       44       -1     

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