Cleanup type conversions in java_bytecode_parsert::read

[tautschnig]

We can safely read signed values and don't need to defer the type conversion to the call site.

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.58%. Comparing base (66004dc) to head (f518b4b).

Files with missing lines	Patch %	Lines
jbmc/src/java_bytecode/java_bytecode_parser.cpp	71.42%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8584      +/-   ##
===========================================
- Coverage    78.71%   78.58%   -0.14%     
===========================================
  Files         1732     1732              
  Lines       199536   199870     +334     
  Branches     18281    18247      -34     
===========================================
+ Hits        157057   157058       +1     
- Misses       42479    42812     +333     

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