Use new package SymbolicAWEModels and remove symbolic code from this package

[ufechner7]

1. Remove the following files:

• mtk_model.jl
• symbolic_awe_model
• system_structure

2. Add SymbolicAWEModels
3. remove obsolete package dependencies
4. test and fix pre-compilation
5. remove obsolete tests
6. remove obsolete documentation

[ufechner7]

Merging this pull request failed, because of the performance:

Pass criteria for the performance on laptop:

First run:

• simplifying the system < 45s

Second run:

• system initialized < 15s
• total time without plotting < 35s

Results on Desktop, 22.09.2025 using the pull request #247

First run:

• simplifying the system: 169s
  Second run:
• system initialized: 16.7 s
• total time without plotting: 36.9s

[1-Bart-1]

@ufechner7 if you want these criteria to be met, you will have to come with a solution yourself. I am ok with the current performance.