Removed manual check of email

New users would never be able to login because of the manual checking of email.
This has happened since the requirements and code has changed over time and we
missed the manual constraint.

This is related to the fix where we removed the database constraint.

src/main/java/se/su/dsv/seshat/services/UserService.java

@@ -17,9 +17,6 @@ public class UserService {
         if (appUserRepository.existsByUsername(username)) {
             throw new IllegalArgumentException("Username already exists");
         }
-        if (appUserRepository.existsByEmail(email)) {
-            throw new IllegalArgumentException("Email already exists");
-        }
 
         AppUser newUser = new AppUser(username, email, "USER");
         appUserRepository.save(newUser);