fix: disable publishing symbol packages (#611)

Seems that there is a problem at the moment: https://twitter.com/fgreinacher/status/1273314597584416768

Author: fgreinacher

appveyor.yml

@@ -29,7 +29,8 @@ before_build:
 
 build:
   publish_nuget: true
-  publish_nuget_symbols: true
+  # Disable symbol publishing until https://twitter.com/fgreinacher/status/1273314597584416768 is resolved.
+  # publish_nuget_symbols: true
 
 before_package:
   # This step replaces the inheritdoc tags in the xml documentation with documentation from mscorlib.xml