remove test_relation(moved to dbt-adapters)

[ChenyuLInx]

#9961

Problem

this test is moved to dbt-adapters already, something changed and now dbt-core CI is broken.
This test was moved to dbt-adapters but not deleted in dbt-core repo. https://github.com/dbt-labs/dbt-adapters/blob/main/tests/unit/test_relation.py

Solution

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.12%. Comparing base (5c8a4ab) to head (575faaf).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10012      +/-   ##
==========================================
- Coverage   88.16%   88.12%   -0.04%     
==========================================
  Files         181      181              
  Lines       22593    22614      +21     
==========================================
+ Hits        19918    19928      +10     
- Misses       2675     2686      +11     

Flag	Coverage Δ
integration	85.44% <ø> (-0.04%)	⬇️
unit	62.21% <ø> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[aranke]

LGTM!