Fix redundant modifier warnings

[MoonBow-1]

Description

This PR "fixes" methods or variables that have redundant modifiers, like final or abstract.

Rationale: https://checkstyle.sourceforge.io/checks/modifier/redundantmodifier.html#RedundantModifier

Does not change the behavior of logic or tests.

Closes #104

Checklists

Testing

General

• I have checked that my test files and functions have meaningful names.
• I have checked that each test tests only a single behavior.
• I have done happy tests.
• I have tested only my own code.
• I have tested at least all public methods.

Assertions

• I have checked that my tests use assertions and not runtime overhead.
• I have checked that my tests end in assertions.
• I have checked that there is no comparison statements in assertions.
• I have checked that assertions are in tests and not in helper functions.
• I have checked that assertions for iterables are outside of for loops and both sides of the iteration blocks.
• I have checked that assertions are not tested inside consumers.

Testing Data

• I have tested algorithms and anything else with the possibility of unbound growth.
• I have checked that all testing data is local and fully replaceable or reproducible or both.
• I have checked that all test files are standalone.
• I have checked that all test-specific fake objects and classes are in the test directory.
• I have checked that my tests do not contain anything related to customers, infrastructure or users.
• I have checked that my tests do not contain non-generic information.
• I have checked that my tests do not do external requests and are not privately or publicly routable.

Statements

• I have checked that my tests do not use throws for exceptions.
• I have checked that my tests do not use try-catch statements.
• I have checked that my tests do not use if-else statements.

Java

• I have checked that my tests for Java uses JUnit library.
• I have checked that my tests for Java uses JUnit utilities for parameters.

Other

• I have only tested public behavior and not private implementation details.
• I have checked that my tests are not (partially) commented out.
• I have checked that hand-crafted variables in assertions are used accordingly.
• I have tested Object Equality.
• I have checked that I do not have any manual tests or I have a valid reason for them and I have explained it in the PR description.

Code Quality

• I have checked that my code follows metrics set in Procedure: Class Metrics.
• I have checked that my code follows metrics set in Procedure: Method Metrics.
• I have checked that my code follows metrics set in Procedure: Object Quality.
• I have checked that my code does not have any NULL values.
• I have checked my code does not contain FIXME or TODO comments.

[eemhu]

Build fails with checkstyle violations.