std_expr.h: use exprt::check consistently

[kroening]

This adds missing calls to the static check method of the expressions subclasses in std_expr.h.

The subsequently unused validate_expr functions are removed.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

❌ Patch coverage is 80.18868% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (0b937eb) to head (f6a9578).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/std_expr.h	80.18%	21 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8831   +/-   ##
========================================
  Coverage    80.00%   80.00%           
========================================
  Files         1700     1700           
  Lines       188345   188248   -97     
  Branches        73       73           
========================================
- Hits        150689   150613   -76     
+ Misses       37656    37635   -21     

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