Fix unset_contextvars to correctly unset the context vars

[gshank]

resolves #82

Description

This is part of the fix for #dbt-core/8866, a problem with node_info persisting when it shouldn't.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0d05e18) 52.23% compared to head (83fd7c5) 54.04%.

Files	Patch %	Lines
dbt_common/events/contextvars.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   52.23%   54.04%   +1.81%     
==========================================
  Files          49       49              
  Lines        2866     2866              
==========================================
+ Hits         1497     1549      +52     
+ Misses       1369     1317      -52     

Flag	Coverage Δ
unit	54.04% <77.77%> (+1.81%)	⬆️

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[emmyoop]

Looks good!