Remove parsert::clear and all of its overrides

[tautschnig]

Re-use of parsers should no longer be necessary now that they aren't objects of static lifetime anymore. Consequently, there is no need to "clear," and instead only a subset of the instructions is required in constructors.

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 79.65%. Comparing base (1fd7011) to head (14715d9).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8244   +/-   ##
========================================
  Coverage    79.64%   79.65%           
========================================
  Files         1684     1683    -1     
  Lines       195606   195549   -57     
========================================
- Hits        155791   155765   -26     
+ Misses       39815    39784   -31     

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