docs: merge cargo-c readme updates

Author: chris-oo

igvm_c/README.md

@@ -19,12 +19,6 @@ API can be built. This can be achieved using:
 cargo install --force cbindgen cargo-c
 ```
 
-Also, `cargo-c` needs to be installed to build the C code:
-
-```bash
-cargo install --force cargo-c
-```
-
 In addition, `sample/dump_igvm` and the C unit tests requires a C compiler to be
 installed.