Remove support for irep_idt-as-std::string

[tautschnig]

While #8040 ensured we were still able to compile the code base with irep_idt typedef'd to std::string (via USE_STD_STRING), experiments at that time also demonstrated that performance is substantially worse, and not all tests would pass.

This commit now removes this untested feature to pave the way for changes that may break dstringt to std::string compatibility.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (69bb2b6) 79.09% compared to head (4d935bf) 79.09%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8154      +/-   ##
===========================================
- Coverage    79.09%   79.09%   -0.01%     
===========================================
  Files         1695     1695              
  Lines       196583   196582       -1     
===========================================
- Hits        155496   155492       -4     
- Misses       41087    41090       +3     

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[martin-cs]

Whatever you think about removing this feature, it should absolutely not be referenced anywhere in analyses/. If anyone wants to veto this patch then these uses still need to be removed.