無神除神, There is no God but The God
So is 無神除神 grammatical correct and good as a translation of the Arabic "La Ilaha Illallah" ("There is no God but the God") 


No to both 

"There is no God but the God" should be "There is no god but God"

The shortened form of "除了(真)神之外沒有神" should be  "除神沒神". But 除 also mean remove, so "除神沒神" could mean "remove God and there will be no God", Therefore, it is not a clear translation for "There is no god but God"

"神外無神" (Beside God, there's no god) is a better translation 

"神外無神"  Short for "真神之外別無他神" (Beside the one true God, there's no other gods)

Seems like there is an existent translation '認主獨一' for  "La Ilaha Illallah" in mainland China.

Since it is for an original design, using an existent official translation actually remove the most important creative element -- the 'creative phrasing' 

As a graphic designer myself, I would have two choices-- one is follow the existent translation, make it a simple 'font' creating project; another is treat it as an original design and 'create' a new phrase. I would choose option 2