Remove duplicate check_chemistry() calls from stage validators

[rayair250-droid]

Addresses part (b) of #1509.

check_chemistry() ran twice per validation: once in validate_common() (which every stage calls) and again at the end of validate_simulation, validate_pre_process, and validate_post_process. Because prohibit() appends to self.errors whenever its condition holds, a single violated chemistry constraint was reported twice. This removes the three redundant per-stage calls; validate_common() already covers all stages, so coverage is unchanged and behavior is identical except that duplicate error-list entries no longer appear.

Smallest-correct-change: 3 deletions, no additions.

On part (a) of the issue (left out on purpose)

I checked the fallback-constant drift against current master and it has partly moved since the issue was filed, so I did not apply the values from the issue:

• The num_ib_patches_max fallback site the issue cited (case_validator.py:585) no longer exists.
• In src/common/m_constants.fpp, num_ib_patches_max is now 2050000 (the issue said 50000).
• The only remaining fallback mismatch is num_patches_max (fallback 1000 vs real 10); num_bc_patches_max already matches.

Since (a) is a never-normally-hit fallback path (hygiene only, as the issue notes) and its premise has drifted, I kept this PR to the clear (b) fix. Happy to do the num_patches_max fallback in a small follow-up if you want it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.43%. Comparing base (66254db) to head (2d1a707).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1627   +/-   ##
=======================================
  Coverage   60.43%   60.43%           
=======================================
  Files          83       83           
  Lines       19868    19868           
  Branches     2956     2956           
=======================================
  Hits        12007    12007           
  Misses       5860     5860           
  Partials     2001     2001           

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