Remove third_party directory

This commit removes the third_party directory, and updates hack/update-deps.sh
to not store licenses in the third_party directory. It also removes symlinks
to third_party in the kodata folders of code in cmd/, which results in third_party
not being included in the images built from these directories.

The reason we have the third_party directory is because some of our dependencies
may have licenses that require distributing source code along with built images.
However, we already include the (compressed) vendor directory in built images
(see the [release pipeline](https://github.com/tektoncd/pipeline/blob/bf26bfb35745b403ac8ca760068f4d102e441940/tekton/publish.yaml#L139-L152)).

The reason to remove third_party is because it is redundant with the vendor directory,
included in unit tests, and CI does not enforce keeping it up to date.
As an alternative to this commit, we could instead enforce that third_party is kept up to date
via hack/verify-codegen.sh.

Makefile

@@ -2,7 +2,7 @@ MODULE   = $(shell env GO111MODULE=on $(GO) list -m)
 DATE    ?= $(shell date +%FT%T%z)
 VERSION ?= $(shell git describe --tags --always --dirty --match=v* 2> /dev/null || \
 			cat $(CURDIR)/.version 2> /dev/null || echo v0)
-PKGS     = $(or $(PKG),$(shell env GO111MODULE=on $(GO) list ./... | grep -v 'github\.com\/tektoncd\/pipeline\/third_party\/'))
+PKGS     = $(or $(PKG),$(shell env GO111MODULE=on $(GO) list ./... ))
 TESTPKGS = $(shell env GO111MODULE=on $(GO) list -f \
 			'{{ if or .TestGoFiles .XTestGoFiles }}{{ .ImportPath }}{{ end }}' \
 			$(PKGS))

hack/update-deps.sh

@@ -58,5 +58,3 @@ if [[ -d hack/patches ]];then
         git apply ${f}
     done
 fi
-
-update_licenses third_party/