remove api that was scheduled for removal

patches/api/Adventure.patch

@@ -459,19 +459,6 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +        this.result = result;
 +    }
 +
-+    /**
-+     * If this decorating is part of a preview request/response.
-+     *
-+     * @return {@code true} if part of previewing
-+     * @deprecated chat preview was removed in 1.19.3
-+     */
-+    @Deprecated(forRemoval = true, since = "1.19.3")
-+    @ApiStatus.ScheduledForRemoval(inVersion = "1.21")
-+    @Contract(value = "-> false", pure = true)
-+    public boolean isPreview() {
-+        return false;
-+    }
-+
 +    @Override
 +    public boolean isCancelled() {
 +        return this.cancelled;