like i said you cant delete the dafult map you can jsut delete OTHER maps. Infact ist a importer bug it should use the dafault map. Its not realy created by maya ist allways there nomatter what. It can be renamed offcourse but its still there.
So again to repeat myself
So first copy the imported map to map 1 then delete the imported map then rename map 1 to be the imported one.
rewnaming it will not make it non default it will and allways will be default nomatter what you do, itcant be deleted! However it can be made into exactly the same thing as you imported map is thus for the users point of view the same thing.