Revert "try using libxmljs built from source in appveyor"

This reverts commit 85bb0bb.

Author: Alban Mouton

appveyor.yml

@@ -8,16 +8,6 @@ install:
   - ps: Install-Product node $env:nodejs_version
   - node --version
   - npm --version
-  - mkdir tmp
-  - cd tmp
-  - git clone https://github.com/albanm/libxmljs.git
-  - cd libxmljs
-  - git submodule init
-  - git submodule update
-  - npm install
-  - npm link
-  - cd ../../
-  - npm link libxmljs-mt
 
 build_script:
   - npm install