Remove out-dated KaTeX part in README

Author: termoshtt

README.md

@@ -86,25 +86,6 @@ Only x86_64 system is supported currently.
 |Netlib   |✔️      |-        |-      |
 |Intel MKL|✔️      |✔️        |✔️      |
 
-Generate document with KaTeX
-------------------------------
-
-You need to set `RUSTDOCFLAGS` explicitly:
-
-```shell
-RUSTDOCFLAGS="--html-in-header katex-header.html" cargo doc --no-deps
-```
-
-This **only** works for `--no-deps` build because `katex-header.html` does not exists for dependent crates.
-If you wish to set `RUSTDOCFLAGS` automatically in this crate, you can put [.cargo/config](https://doc.rust-lang.org/cargo/reference/config.html):
-
-```toml
-[build]
-rustdocflags = ["--html-in-header", "katex-header.html"]
-```
-
-But, be sure that this works only for `--no-deps`. `cargo doc` will fail with this `.cargo/config`.
-
 License
 --------