drop support for --csr-data-width=8

[maass-hamburg]

with a2b71fd 32 bit csr data width was made the default, that was 5 years ago. 8-bit support was only left in code for compatibility, but it wasn't really maintained.

For example

litex_sim --csr-data-width=8

will not boot.

Therefore it might be time to drop support.