Disable SBT options

Some weird error happens on Appveyor
https://ci.appveyor.com/project/liufengyun/dotty/builds/20616903

Author: liufengyun

.appveyor.yml

@@ -3,10 +3,9 @@ os: Windows Server 2012
 install:
   - cmd: choco install sbt -ia "INSTALLDIR=""C:\sbt"""
   - cmd: SET PATH=C:\MinGW\msys\1.0\bin;C:\sbt\bin;%JAVA_HOME%\bin;%PATH%
-  - cmd: SET SBT_OPTS=-XX:MaxPermSize=2g -Xmx4g
+  # - cmd: SET SBT_OPTS=-XX:MaxPermSize=2g -Xmx4g
 clone_script:
   - pwd
-  - ls
   - git clone --depth 50 https://github.com/lampepfl/dotty.git
   - cd dotty
   - git submodule update --init --recursive --depth 50 --jobs 3