Revert MRUBY_TARGET mechanism; ref #5096

* In explanation of mruby, the expression `build_config.rb` is frequently
  used including official documents, so I think that it will not make sense
  if the file is no longer used.
* The `MRUBY_TARGET` mechanism seems to have little improvement, so I don't
  think it should be changed to avoid unnecessary confusion.
* `MRUBY_TARGET` and `MRuby.targets` represent somewhat different things,
  so using the same term "target" is a bit confusing.

The mechanism that can be written short when using a file under
`build_config` (renamed from `target`) directory remains
(`build_config/${MRUBY_CONFIG}.rb` is used if the path specified
in `MRUBY_CONFIG` doesn't exist).

tasks/benchmark.rake

@@ -9,7 +9,11 @@ def bm_files
 end
 
 def build_config_name
-  MRUBY_TARGET
+  if !ENV['MRUBY_CONFIG'].to_s.empty?
+    File.basename(ENV['MRUBY_CONFIG'], '.rb').gsub('build_config_', '')
+  else
+    "build"
+  end
 end
 
 def plot_file