build: make rclone build with wasm

Needed to drop
- azureblob backend
- cache backend
- qingstor backend
- cachestats command
- ncdu command

cmd/ncdu/ncdu.go

@@ -1,6 +1,6 @@
 // Package ncdu implements a text based user interface for exploring a remote
 
-//+build !plan9,!solaris
+//+build !plan9,!solaris,!js
 
 package ncdu
 

cmd/ncdu/ncdu_unsupported.go

@@ -1,6 +1,6 @@
 // Build for ncdu for unsupported platforms to stop go complaining
 // about "no buildable Go source files "
 
-// +build plan9 solaris
+// +build plan9 solaris js
 
 package ncdu