Remove qualifierst

[tautschnig]

We only ever inherit from c_qualifierst and can safely avoid the use of dynamic_cast by making expr2ct use c_qualifierst instead of qualifierst. The clone() facility introduced in #1831 (where qualifierst was introduced) remains in place to support derived implementations (like java_qualifierst).

Also, reduce the number of explicit c_qualifierst() calls by making use of convert and convert_with_identifier.

• 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.
• 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

Attention: Patch coverage is 62.96296% with 10 lines in your changes missing coverage. Please review.

Project coverage is 77.95%. Comparing base (3877e0f) to head (eabcf9c).
Report is 2 commits behind head on develop.

Files	Patch %	Lines
src/ansi-c/c_qualifiers.h	33.33%	4 Missing ⚠️
src/cpp/expr2cpp.cpp	33.33%	4 Missing ⚠️
src/ansi-c/expr2c.cpp	80.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8419      +/-   ##
===========================================
+ Coverage    77.94%   77.95%   +0.01%     
===========================================
  Files         1726     1726              
  Lines       189621   189591      -30     
  Branches     18247    18276      +29     
===========================================
- Hits        147792   147791       -1     
+ Misses       41829    41800      -29     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.