Remove legacy renderer from source, build files and CI (#3384)

.github/scripts/windows-ci_configure_wrapper.ps1

@@ -12,12 +12,6 @@ switch ($env:RENDERER)
 	'osmesa' { $compile_flags += '-DMLN_WITH_OSMESA=ON'; break; }
 }
 
-switch ($env:RENDERING_MODE)
-{
-	'legacy'   { $compile_flags += '-DMLN_LEGACY_RENDERER=ON'  ; break; }
-	'drawable' { $compile_flags += '-DMLN_DRAWABLE_RENDERER=ON'; break; }
-}
-
 Write-Host 'Compile flags: ' $compile_flags
 
 & cmake -B build -G Ninja $($compile_flags)