[v8] Remove deprecated Templates API

[arnels]

v8 overview: https://cognitedata.atlassian.net/browse/DM-3055

Description

Templates was deprecated in v7, released 2023-11-14. The Templates API itself was retired and turned off 2025-05-31. Time to remove the unused code.

To be included in v8 release #2118. The Templates API is turned off, but removing the code from the SDK in a minor release could potentially break clients.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.07%. Comparing base (500e257) to head (12878e9).
⚠️ Report is 74 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2251      +/-   ##
==========================================
+ Coverage   90.81%   91.07%   +0.26%     
==========================================
  Files         166      164       -2     
  Lines       24636    24269     -367     
==========================================
- Hits        22373    22104     -269     
+ Misses       2263     2165      -98     

Files with missing lines	Coverage Δ
cognite/client/_cognite_client.py	93.93% <ø> (-0.13%)	⬇️
cognite/client/data_classes/__init__.py	100.00% <ø> (ø)
cognite/client/testing.py	100.00% <ø> (ø)

... and 4 files with indirect coverage changes

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[haakonvt]

Closing as merged manually into #2273