Cleanup validation of reference_typet

[tautschnig]

validate_type is not actually used (anymore), instead static member
functions check are used.

• 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

Merging #5658 (71af819) into develop (f85b11d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5658   +/-   ##
========================================
  Coverage    69.41%   69.42%           
========================================
  Files         1243     1243           
  Lines       100621   100627    +6     
========================================
+ Hits         69850    69856    +6     
  Misses       30771    30771           

Flag	Coverage Δ
cproversmt2	43.14% <0.00%> (-0.01%)	⬇️
regression	66.31% <100.00%> (+<0.01%)	⬆️
unit	32.27% <57.14%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/util/std_types.h	94.46% <100.00%> (+0.06%)	⬆️
src/util/validate_types.cpp	88.46% <100.00%> (+0.96%)	⬆️

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