docs: remove mention that buildkit warns about unconsumned build args

BuildKit doesn't warn if a provided build argument is not consumed,
unlike the classic builder.

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

frontend/dockerfile/docs/reference.md

@@ -2098,14 +2098,6 @@ flag.
 > learn about secure ways to use secrets when building images.
 { .warning }
 
-
-If you specify a build argument that wasn't defined in the Dockerfile,
-the build outputs a warning.
-
-```console
-[Warning] One or more build-args [foo] were not consumed.
-```
-
 A Dockerfile may include one or more `ARG` instructions. For example,
 the following is a valid Dockerfile: