fix: Remove nonsensical duration variance

[ritchie46]

The unit of a variance is input unit squared.

So a variance of data in m is m^2. Given that we don't have a datatype with s^2, doing a variance on duration is non-sensical.

This will remove that implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 27.27273% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.37%. Comparing base (a939b7b) to head (ebd1fc9).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...es/polars-ops/src/chunked_array/list/dispersion.rs	0.00%	5 Missing ⚠️
...tes/polars-ops/src/chunked_array/list/namespace.rs	0.00%	1 Missing ⚠️
.../polars-plan/src/plans/aexpr/function_expr/list.rs	0.00%	1 Missing ⚠️
...olars-plan/src/plans/aexpr/function_expr/schema.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23621      +/-   ##
==========================================
- Coverage   81.41%   81.37%   -0.05%     
==========================================
  Files        1640     1640              
  Lines      222402   222378      -24     
  Branches     2836     2836              
==========================================
- Hits       181068   180959     -109     
- Misses      40635    40720      +85     
  Partials      699      699              

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[github-actions]

The uncompressed lib size after this PR is 42.1696 MB.