Do not use caching repository by default

The caching repository does not work universally so set the default to
always poll. This is still faster than in sbt 1.2.x because of
performance improvements that I added for listing directories.

main/src/main/scala/sbt/Defaults.scala

@@ -282,10 +282,7 @@ object Defaults extends BuildCommon {
         None
       },
       watchStartMessage := Watched.defaultStartWatch,
-      fileTreeRepository := state.value
-        .get(Keys.globalFileTreeRepository)
-        .map(FileTree.repository)
-        .getOrElse(FileTree.Repository.polling),
+      fileTreeRepository := FileTree.Repository.polling,
       externalHooks := {
         val repository = fileTreeRepository.value
         compileOptions =>