Remove deprecated messaget() constructor

[tautschnig]

It is now no longer possible to construct a messaget object that is not fully configured, i.e., lacks a message handler.

• 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 78.40%. Comparing base (8bab263) to head (8110e31).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8143      +/-   ##
===========================================
+ Coverage    78.23%   78.40%   +0.17%     
===========================================
  Files         1722     1722              
  Lines       188692   188260     -432     
  Branches     18452    18473      +21     
===========================================
- Hits        147615   147612       -3     
+ Misses       41077    40648     -429     

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