build: make rclone build with wasm

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

backend/azureblob/azureblob_unsupported.go

@@ -1,6 +1,6 @@
 // Build for azureblob for unsupported platforms to stop go complaining
 // about "no buildable Go source files "
 
-// +build plan9 solaris !go1.13
+// +build plan9 solaris js !go1.13
 
 package azureblob