chore: get rid of k8s.io/kubernetes dep

This dependency is problematic because of the many replaces it brings
with it. This makes it hard to import external-snapshotter in 3rd
party projects without having a lot of `replace` directives in those
projects as well.

The only usage of that import was to get two very simple utility
functions so I copied them over to this repo.

vendor/k8s.io/kubernetes/pkg/util/slice/BUILD

@@ -1,32 +0,0 @@
-package(default_visibility = ["//visibility:public"])
-
-load(
-    "@io_bazel_rules_go//go:def.bzl",
-    "go_library",
-    "go_test",
-)
-
-go_library(
-    name = "go_default_library",
-    srcs = ["slice.go"],
-    importpath = "k8s.io/kubernetes/pkg/util/slice",
-)
-
-go_test(
-    name = "go_default_test",
-    srcs = ["slice_test.go"],
-    embed = [":go_default_library"],
-)
-
-filegroup(
-    name = "package-srcs",
-    srcs = glob(["**"]),
-    tags = ["automanaged"],
-    visibility = ["//visibility:private"],
-)
-
-filegroup(
-    name = "all-srcs",
-    srcs = [":package-srcs"],
-    tags = ["automanaged"],
-)

vendor/k8s.io/kubernetes/pkg/util/slice/slice.go

@@ -1,75 +0,0 @@
-/*
-Copyright 2015 The Kubernetes Authors.
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-    http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-*/
-
-// Package slice provides utility methods for common operations on slices.
-package slice
-
-import (
-	"sort"
-)
-
-// CopyStrings copies the contents of the specified string slice
-// into a new slice.
-func CopyStrings(s []string) []string {
-	if s == nil {
-		return nil
-	}
-	c := make([]string, len(s))
-	copy(c, s)
-	return c
-}
-
-// SortStrings sorts the specified string slice in place. It returns the same
-// slice that was provided in order to facilitate method chaining.
-func SortStrings(s []string) []string {
-	sort.Strings(s)
-	return s
-}
-
-// ContainsString checks if a given slice of strings contains the provided string.
-// If a modifier func is provided, it is called with the slice item before the comparation.
-func ContainsString(slice []string, s string, modifier func(s string) string) bool {
-	for _, item := range slice {
-		if item == s {
-			return true
-		}
-		if modifier != nil && modifier(item) == s {
-			return true
-		}
-	}
-	return false
-}
-
-// RemoveString returns a newly created []string that contains all items from slice that
-// are not equal to s and modifier(s) in case modifier func is provided.
-func RemoveString(slice []string, s string, modifier func(s string) string) []string {
-	newSlice := make([]string, 0)
-	for _, item := range slice {
-		if item == s {
-			continue
-		}
-		if modifier != nil && modifier(item) == s {
-			continue
-		}
-		newSlice = append(newSlice, item)
-	}
-	if len(newSlice) == 0 {
-		// Sanitize for unit tests so we don't need to distinguish empty array
-		// and nil.
-		newSlice = nil
-	}
-	return newSlice
-}