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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• 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.
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[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.37%. Comparing base (41e9f3d) to head (d0e1b69).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
jbmc/src/java_bytecode/java_bytecode_parser.cpp	80.00%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8584      +/-   ##
===========================================
- Coverage    80.38%   80.37%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       206770   206785      +15     
  Branches        85       85              
===========================================
- Hits        166207   166205       -2     
- Misses       40563    40580      +17     

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[peterschrammel]

What motivated this cleanup?