csplit: remove crate_name attribute

[cakebaker]

While reviewing #6090 I noticed a crate_name attribute I think is unnecessary as no other util uses it. According to https://doc.rust-lang.org/rust-by-example/attribute/crate.html:

it is important to note that both the crate_type and crate_name attributes have no effect whatsoever when using Cargo, the Rust package manager.

[sylvestre]

TIL

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/dd/misc. tests/dd/misc is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/csplit/csplit is no longer failing!