Remove legacy code from php-cs-fixer

[nlubisch]

1. Why is this change necessary?

2. What does this change do, exactly?

3. Describe each step to reproduce the issue or behaviour.

4. Please link to the relevant issues (if any).

5. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code

[teiling88]

pls don't remove it

[codecov]

Codecov Report

Merging #756 into development will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             development    #756   +/-   ##
=============================================
  Coverage           4.99%   4.99%           
  Complexity          2921    2921           
=============================================
  Files                262     262           
  Lines              16965   16965           
=============================================
  Hits                 848     848           
  Misses             16117   16117

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