Remove useless \simple markup command
As the edits to Documentation/*/extending/programming-interface.itely show, it apparently used to be in old times that \markup "a" would not be understood as #"a" but #(make-simple-markup "a"). This is no longer the case. The \simple markup command is now useless. \markup \simple "a" should just become \markup "a".
Edited by Jean Abou Samra