Revert "Check if the found pkg-config is usable actually"

This reverts commit 79d8a31.

The second argument of `find_executable0` in mkmf is `path`, not
arguments to the program like as `EnvUtil.find_executable`.

Author: nobu

test/mkmf/test_pkg_config.rb

@@ -3,9 +3,7 @@
 require 'shellwords'
 
 class TestMkmfPkgConfig < TestMkmf
-  PKG_CONFIG = config_string("PKG_CONFIG") do |path|
-    find_executable0(path, "--version") {$?.success?}
-  end
+  PKG_CONFIG = config_string("PKG_CONFIG") {|path| find_executable0(path)}
 
   def setup
     super