Update documentation of Debian dependencies

Bindgen doesn't need the llvm package at all and only needs the clang package for one function. I've tested this by running the bindgen tests on a fresh Debian installation.

I removed the line about Ubuntu because it is not relevant anymore.

closes #2934

book/src/requirements.md

@@ -48,10 +48,10 @@ $ port install clang
 #### Debian-based Linuxes
 
 ```bash
-# apt install llvm-dev libclang-dev clang
+# apt install libclang-dev
 ```
 
-Ubuntu 18.04 provides the necessary packages directly.
+If you want to use the function `bindgen::Builder::dump_preprocessed_input`, then you also need the package `clang`.
 
 #### Arch