Deletes text between _start_ and _end_ position.
Supprime un texte compris entre les positions _start_ et _end_.
_buffer_ : ObjGtkTextBuffer : any textbuffer object already created.
_start_ : I : the start position, counting from 0. If equal at nil, the position is 0 (before the first character).
_end_ : I : the stop position, counting from 0. If equal at nil, the position is after the last character.
Return : ObjGtkTextBuffer : the same textbuffer or nil if an error occurs.