meta: Remove support for multisample blits

meta: Remove support for multisample blits

Since i965 no longer uses this function for blitting color buffers,
there is no driver left that will ever support multisample textures and
use _mesa_meta_BlitFramebuffer.

v2: Delete blit_state::msaa_shaders and enum blit_msaa_shader.

   text     data     bss      dec    hex  filename
12243286 1344936 1290748 14878970 e308fa  before/lib64/dri/i965_dri.so
12240398 1344936 1290748 14876082 e2fdb2  after/lib64/dri/i965_dri.so

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/856>