ci(azure-pipelines): XXX Don't install libxml2 with homebrew

Seems like that shouldn't be done fore recent macOS versions.

> libxml2 is keg-only, which means it was not symlinked into /usr/local,
> because macOS already provides this software and installing another version in
> parallel can cause all kinds of trouble.

.azure-pipelines/steps/setup-emacs.yml

@@ -22,7 +22,7 @@ steps:
 # XXX: Please don't use homebrew!
 - bash: |
     set -e
-    brew install pkg-config gnutls libxml2
+    brew install pkg-config gnutls
     # https://github.com/remacs/remacs/issues/155 (pkg-config)
     brew outdated pkg-config || brew upgrade pkg-config
     # https://github.com/remacs/remacs/issues/886 (libxml2, macOS 10.14)