move Environment#shell to BasicEnvironment; clean up a few yard warnings

lib/rscons/environment.rb

@@ -487,29 +487,6 @@ module Rscons
       end
     end
 
-    # Execute a command using the system shell.
-    #
-    # The shell is automatically determined but can be overridden by the SHELL
-    # construction variable. If the SHELL construction variable is specified,
-    # the flag to pass to the shell is automatically dtermined but can be
-    # overridden by the SHELLFLAG construction variable.
-    #
-    # @param command [String] Command to execute.
-    #
-    # @return [String] The command's standard output.
-    def shell(command)
-      shell_cmd =
-        if shell = get_var("SHELL")
-          flag = get_var("SHELLFLAG") || (shell == "cmd" ? "/c" : "-c")
-          [shell, flag]
-        else
-          Rscons.get_system_shell
-        end
-      IO.popen([*shell_cmd, command]) do |io|
-        io.read
-      end
-    end
-
     # Print the builder run message, depending on the Environment's echo mode.
     #
     # @param builder [Builder]