Remove deprecated "piwik"

As noted in https://github.com/docker-library/docs/pull/1206/files#diff-d161a79ef9b88daccb45cd0f03f1c4bd, this will receive no further updates and users are encouraged to use `matomo` instead.

library/piwik

@@ -1,18 +0,0 @@
-# This file is generated via https://github.com/matomo-org/docker/blob/d1f85155be775e20bd66346db2ea5025d40e275d/generate-stackbrew-library.sh
-Maintainers: Pierre Ozoux <[email protected]> (@pierreozoux)
-GitRepo: https://github.com/matomo-org/docker.git
-
-Tags: 3.6.0-apache, 3.6-apache, 3-apache, apache, 3.6.0, 3.6, 3, latest
-Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
-GitCommit: b711c66be5de3caade38361bd974134e1291a929
-Directory: apache
-
-Tags: 3.6.0-fpm, 3.6-fpm, 3-fpm, fpm
-Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
-GitCommit: b711c66be5de3caade38361bd974134e1291a929
-Directory: fpm
-
-Tags: 3.6.0-fpm-alpine, 3.6-fpm-alpine, 3-fpm-alpine, fpm-alpine
-Architectures: amd64, arm32v6, arm64v8, i386, ppc64le, s390x
-GitCommit: b711c66be5de3caade38361bd974134e1291a929
-Directory: fpm-alpine