I don't know if it's the right way to do it, but the following worked for me.
I opened the xkb/symbols/altwin file, and replaced the line:
key <RALT> { type[Group1] = "TWO_LEVEL"; symbols[Group1] = [ Alt_R, Meta_R ] };
by:
key <RALT> { type[Group1] = "ONE_LEVEL"; symbols[Group1] = [ Alt_R ] };
I don't know if it's the right way to do it, but the following worked for me.
symbols[ Group1] = [ Alt_R, Meta_R ] };
symbols[ Group1] = [ Alt_R ] };
I opened the xkb/symbols/altwin file, and replaced the line:
key <RALT> { type[Group1] = "TWO_LEVEL";
by:
key <RALT> { type[Group1] = "ONE_LEVEL";
PS : Commenting the line works, too.