updated validate method and removed isValid property

src/main/java/io/apimatic/coreinterfaces/authentication/Authentication.java

@@ -7,11 +7,6 @@
  */
 public abstract class Authentication {
 
-    /**
-     * Stores the validity of the auth scheme.
-     */
-    private boolean isValid;
-
     /**
      * Stores the error message for the auth scheme.
      */
@@ -26,24 +21,9 @@ public abstract class Authentication {
 
     /**
      * Validates the credentials for authentication.
-     */
-    public abstract void validate();
-
-    /**
-     * Checks if the auth credentials are valid.
      * @return true if the auth credentials are valid, false otherwise.
      */
-    public boolean isValid() {
-        return isValid;
-    }
-
-    /**
-     * Sets the validatity of the auth credentials.
-     * @param isValid the flag to set for validity.
-     */
-    public void setValidity(boolean isValid) {
-        this.isValid = isValid;
-    }
+    public abstract boolean validate();
 
     /**
      * Returns the error message if the auth credentials are not valid.