Remove SequentFormula class

[unp1]

Intended Change

This PR prepares the ground for generalizing proof and calculus structures.
It removes the superfluous SequentFormula class, which is only a behaviorless
container for a term. The class is a left-over from times when we had formulas with
constraints on the sequent level

It depends on PR #3472 to be merged first as it is based on that branch

Plan

• Pass tests
• Code cleanup

Type of pull request

• Refactoring (behaviour should not change or only minimally change)

Ensuring quality

• I have tested the feature as follows: tests
• I have checked that runtime performance has not deteriorated.

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.

[codecov]

Codecov Report

Attention: Patch coverage is 52.52918% with 366 lines in your changes missing coverage. Please review.

Project coverage is 38.03%. Comparing base (345c9c6) to head (fca7bd8).
Report is 46 commits behind head on main.

Files	Patch %	Lines
...java/de/uka/ilkd/key/proof/join/JoinProcessor.java	0.00%	16 Missing ⚠️
...ore/src/main/java/de/uka/ilkd/key/rule/Taclet.java	30.43%	13 Missing and 3 partials ⚠️
...ka/ilkd/key/macros/scripts/InstantiateCommand.java	0.00%	12 Missing ⚠️
.../de/uka/ilkd/key/macros/AbstractBlastingMacro.java	0.00%	11 Missing ⚠️
...core/src/main/java/org/key_project/logic/Term.java	0.00%	11 Missing ⚠️
.../src/main/java/de/uka/ilkd/key/smt/SMTProblem.java	0.00%	8 Missing ⚠️
...ey/strategy/feature/InfFlowContractAppFeature.java	0.00%	8 Missing ⚠️
...de/uka/ilkd/key/macros/scripts/RewriteCommand.java	41.66%	5 Missing and 2 partials ⚠️
...e/uka/ilkd/key/rule/UseDependencyContractRule.java	22.22%	7 Missing ⚠️
...src/main/java/de/uka/ilkd/key/logic/JavaBlock.java	0.00%	6 Missing ⚠️
... and 112 more

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3478      +/-   ##
============================================
- Coverage     38.03%   38.03%   -0.01%     
- Complexity    17084    17099      +15     
============================================
  Files          2099     2105       +6     
  Lines        127194   127378     +184     
  Branches      21368    21389      +21     
============================================
+ Hits          48375    48443      +68     
- Misses        72856    72972     +116     
  Partials       5963     5963              

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