[bundler/bundler] Revert "make system_bundle_bin_path helper and resolve failing tests for ruby < 2.6"

This reverts commit e63e844bc7444c6a489fcde0dc7011c6c4807edd.

It was introduced to resolve some failing tests at the cost of making
the intention of the spec much less clear.

Thanks to the previous fixes we have added to this spec, we can revert
that patch now.

https://github.com/bundler/bundler/commit/b29a40820f

spec/bundler/commands/exec_spec.rb

@@ -856,7 +856,7 @@ __FILE__: #{path.to_s.inspect}
         file = bundled_app("file_that_bundle_execs.rb")
         create_file(file, <<-RB)
           #!#{Gem.ruby}
-          puts `#{system_bundle_bin_path} exec echo foo`
+          puts `bundle exec echo foo`
         RB
         file.chmod(0o777)
         bundle! "exec #{file}", :system_bundler => true

spec/bundler/support/helpers.rb

@@ -110,7 +110,7 @@ module Spec
       bundle_bin = options.delete("bundle_bin") || bindir.join("bundle")
 
       if system_bundler = options.delete(:system_bundler)
-        bundle_bin = system_bundle_bin_path
+        bundle_bin = system_gem_path.join("bin/bundler")
       end
 
       env = options.delete(:env) || {}

spec/bundler/support/path.rb

@@ -100,10 +100,6 @@ module Spec
       tmp("gems/system", *path)
     end
 
-    def system_bundle_bin_path
-      system_gem_path("bin/bundle")
-    end
-
     def lib_path(*args)
       tmp("libs", *args)
     end