Remove unused pSpatialLayer variable in DetermineTemporalSettings

[fippo]

which breaks on Mac CI settings due to the variable being unused

Fixes the build issue from #3752, as a separate commit for papertrail reasons.

./codec/encoder/core/inc/param_svc.h:496:26: error: variable 'pSpatialLayer' set but 
not used [-Werror,-Wunused-but-set-variable]
    SSpatialLayerConfig* pSpatialLayer = &sSpatialLayers[0];

[fippo]

looks like the Mac defaults changed? WDYT about adding

  '-Werror',
  '-Wunused-but-set-variable',

to supported_arguments in the meson file?

[BenzhengZhang]

mac build failed, test/decoder/DecUT_DecExt.cpp:143:11: error: variable 'iSliceIndex' set but not used [-Werror,-Wunused-but-set-variable] @fippo

[fippo]

Thank you, there was more after that even (and oddly adding those compiler defines to the linux build did not help).
I did a "fork" of the repo to get the CI jobs running and now https://github.com/fippo/openh264-mac-build/actions/runs/9614665797 passes for me