subsample: Don't validate schema in helper functions

[victorlin]

Description of proposed changes

This change is primarily intended for pathogen workflows that use the helper functions prior to running augur subsample, commonly during Snakemake's initialization step. Schema is still validated at run time.

We are shifting to a pattern where pathogen workflows define and validate their own schema at workflow start, so the validation in these helper functions is unnecessary duplicate work.

Related issue(s)

Example with measles:

Saved 'config' to 'results/run_config.yaml'.
Validating schema of 'results/run_config.yaml'...                             ← validation already happens here
Saved 'config.subsample.genome/global' to 'results/genome/global/subsample_config.yaml'.
Saved 'config.subsample.N450/global' to 'results/N450/global/subsample_config.yaml'.
Saved 'config.subsample.genome/north-america' to 'results/genome/north-america/subsample_config.yaml'.
host: b1f8e2df1311
Building DAG of jobs...
Validating schema of 'results/genome/global/subsample_config.yaml'...         ← duplicate validation
Validating schema of 'results/N450/global/subsample_config.yaml'...           ← duplicate validation
Validating schema of 'results/genome/north-america/subsample_config.yaml'...  ← duplicate validation

Checklist

• Automated checks pass failures unrelated
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.26%. Comparing base (405c162) to head (8752942).

Files with missing lines	Patch %	Lines
augur/subsample.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2039   +/-   ##
=======================================
  Coverage   73.25%   73.26%           
=======================================
  Files          86       86           
  Lines       10766    10765    -1     
  Branches     2099     2099           
=======================================
  Hits         7887     7887           
+ Misses       2504     2503    -1     
  Partials      375      375           

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