♻️ remove redundant start parameter from DD multiplication routine

[burgholzer]

Description

This small PR removes a no-longer-needed start parameter of the DD multiplication routines.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.2%. Comparing base (df43b4a) to head (f06e8fd).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #576   +/-   ##
=====================================
  Coverage   91.2%   91.2%           
=====================================
  Files        132     132           
  Lines      13904   13904           
  Branches    2203    2203           
=====================================
  Hits       12692   12692           
  Misses      1212    1212           

Flag	Coverage Δ
cpp	90.9% <100.0%> (ø)
python	99.5% <ø> (ø)

Files	Coverage Δ
include/mqt-core/dd/Package.hpp	97.6% <100.0%> (ø)