AppVeyor: Drop "clone_depth: 1" because subsequent pushes were failing ongoing builds

Symptom:
  Build started
  git config --global core.autocrlf input
  git clone -q --depth=1 --branch=master https://github.com/libexpat/libexpat.git c:\projects\libexpat
  git checkout -qf d4d8d486af
  fatal: reference is not a tree: d4d8d486af

Cause:
  A more recent push made d4d8d486af
  the second-last commit so that --depth=1 would create a commit
  graph without that commit.

Documented at:
https://www.appveyor.com/docs/how-to/repository-shallow-clone/#setting-depth-of-git-clone-command

appveyor.yml

@@ -36,9 +36,6 @@ init:
 # version format
 version: libexpat-{build}
 
-# set clone depth, clone entire repository history if not defined
-clone_depth: 1
-
 # clone directory
 clone_folder: c:\projects\libexpat