handle single command and remove hacks used as substitute for it

mpd-git has single command, which provides functionality
of both "repeat one" and "stop after current track" modes.

repeat one = single && repeat
stop after current track = single && !repeat

doc/config

@@ -124,8 +124,6 @@
 #
 #autocenter_mode = "no"
 #
-#repeat_one_mode = "no"
-#
 #default_place_to_search_in = "database" (database/playlist)
 #
 #media_library_left_column = "a" (possible values: a,y,g,c,p, legend above)