build: remove build constraint syntax for go 1.16 and older

backend/qingstor/qingstor.go

@@ -1,5 +1,4 @@
 //go:build !plan9 && !js
-// +build !plan9,!js
 
 // Package qingstor provides an interface to QingStor object storage
 // Home: https://www.qingcloud.com/

backend/qingstor/qingstor_test.go

@@ -1,7 +1,6 @@
 // Test QingStor filesystem interface
 
 //go:build !plan9 && !js
-// +build !plan9,!js
 
 package qingstor
 

backend/qingstor/qingstor_unsupported.go

@@ -2,6 +2,5 @@
 // about "no buildable Go source files "
 
 //go:build plan9 || js
-// +build plan9 js
 
 package qingstor

backend/qingstor/upload.go

@@ -1,7 +1,6 @@
 // Upload object to QingStor
 
 //go:build !plan9 && !js
-// +build !plan9,!js
 
 package qingstor