remove deprecated builder/dockerignore

This package was deprecated in de56a90929c5cde9697fc7c83f379cbddb21e323, which
was part of the 20.10 release, so consumers of this package should've been
able to migrate to the new location.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerignore/deprecated.go

@@ -1,17 +0,0 @@
-// Package dockerignore is deprecated. Use github.com/moby/buildkit/frontend/dockerfile/dockerignore instead.
-package dockerignore
-
-import (
-	"io"
-
-	"github.com/moby/buildkit/frontend/dockerfile/dockerignore"
-)
-
-// ReadAll reads a .dockerignore file and returns the list of file patterns
-// to ignore. Note this will trim whitespace from each line as well
-// as use GO's "clean" func to get the shortest/cleanest path for each.
-//
-// Deprecated: use github.com/moby/buildkit/frontend/dockerfile/dockerignore.ReadAll instead.
-func ReadAll(reader io.Reader) ([]string, error) {
-	return dockerignore.ReadAll(reader)
-}