providers/mock.go: Remove copyright headers

This was probably added accidentally.

The removal aligns with this ignore pattern entry in the .copywrite.hcl config: "**/mock*.go"

internal/providers/mock.go

@@ -1,6 +1,3 @@
-// Copyright (c) HashiCorp, Inc.
-// SPDX-License-Identifier: BUSL-1.1
-
 package providers
 
 import (