st/mesa: remove some old GetTexImage() code
st/mesa: remove some old GetTexImage() code

We can use the core Mesa code for glGetTexImage() since it handles the
image mapping/unmapping now.  We'll keep the decompress_with_blit() path
in the hope that it's faster than core Mesa's software decompression code.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=41312

djreep81/glsl-optimizer,mapbox/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,wolf96/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,mapbox/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,tokyovigilante/glsl-optimizer