I have re arranged the sign up form and worked out how to hide the City field and the alert. What I have done is on the configuration table I have changed value of the ENTRY_CITY_MIN_LENGTH to 0.
It works well with no alert.
However when I am updating Address Detail and click on update I get the message "Your City must contain a minimum of 0 characters"
I also found this alert can appear on other modules.
Is there any idea how to clear the alert and make the city field not compulsory from all modules?