pkg-config is no longer necessary

Author: akihikodaki

README.md

@@ -44,8 +44,6 @@ https://github.com/jruby/jruby/pull/4118/commits/edad375ef4dcf195b19ce0afe4befac
 `gem install cld3` triggers native library building. If it fails, it is likely
 that some required facilities are missing. Make sure C++ compiler is installed.
 I recommend [GCC](https://gcc.gnu.org/) as a C++ compiler.
-Ruby is likely to need [pkg-config](https://www.freedesktop.org/wiki/Software/pkg-config/)
-as well.
 
 Runtime errors are likely to be issues of [FFI](https://github.com/ffi/ffi) or
 programming errors. Make sure they are all correct.