Remove surface_integral from prolong2interfaces

[benegee]

Xref: trixi-framework/Trixi.jl#2434

This was not caught in Trixi.jl's downstream tests as we currently only run one Euler elixir.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.68%. Comparing base (8e9054c) to head (080581a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage   90.68%   90.68%           
=======================================
  Files          22       22           
  Lines        2189     2189           
=======================================
  Hits         1985     1985           
  Misses        204      204           

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