Fix flux for quasi-1D SWE

[jlchan]

Remove the non-conservative pressure term in the quasi-1D SWE flux. This fixes a bug in flux_lax_friedrichs.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6158ba4) 89.28% compared to head (c0f859f) 92.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1731      +/-   ##
==========================================
+ Coverage   89.28%   92.00%   +2.72%     
==========================================
  Files         431      432       +1     
  Lines       34641    34687      +46     
==========================================
+ Hits        30928    31911     +983     
+ Misses       3713     2776     -937     

Flag	Coverage Δ
unittests	92.00% <100.00%> (+2.72%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...gsem/elixir_shallowwater_quasi_1d_discontinuous.jl	100.00% <100.00%> (ø)
src/equations/shallow_water_quasi_1d.jl	93.40% <100.00%> (+7.29%)	⬆️

... and 42 files with indirect coverage changes

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[sloede]

LGTM technically, but someone from the SWE crew should have a look at the flux implementation

[jlchan]

Failing parabolic tests should be fixed by #1728.

[ranocha]

Thanks! I plan to backport your fix first in #1732, then update this PR, merge it, release a new version v0.5.x of Trixi.jl with all tests passing except the neural network stuff, and finally merge #1705 (in a regular merge, not a squash merge) to release v0.6.0 of Trixi.jl.