Remove default OpenMW shader files that aren't changed

[AbduSharif]

This would prevent this shader pack from becoming outdated quickly whenever a new change happens to irrelevant shader files that the pack doesn't change at all when compared to OpenMW's default shaders.

[AbduSharif]

Examples of changes that the shader pack is missing: https://gitlab.com/OpenMW/openmw/-/commits/master/files/shaders

Doing this should made things easier to deal with.