Rely on the build environment rather than use MacOS.locate

Our build environment setup ensures these tools will be on the PATH,
so we shouldn't use private API to find them.

Formula/ispell.rb

@@ -25,7 +25,7 @@ class Ispell < Formula
     chmod 0644, "config.sh"
     inreplace "config.sh" do |s|
       s.gsub! '/usr/share/dict', "#{share}/dict"
-      s.gsub! /yacc/, MacOS.locate('yacc')
+      s.gsub! /yacc/, "yacc"
     end
 
     (lib/'ispell').mkpath

Formula/libmxml.rb

@@ -14,7 +14,7 @@ class Libmxml < Formula
                           "--prefix=#{prefix}"
 
     # Makefile hard-codes the path to /Developer
-    inreplace "Makefile", "/Developer/usr/bin/docsetutil", MacOS.locate('docsetutil')
+    inreplace "Makefile", "/Developer/usr/bin/docsetutil", "docsetutil"
 
     system "make"
     system "make install"