syscall: remove unused variable

Author: dgryski

src/syscall/libc_wasip2.go

@@ -107,7 +107,6 @@ type wasiFile struct {
 //   The mismatch comes from trying to combine these.
 
 var wasiFiles map[int32]*wasiFile = make(map[int32]*wasiFile)
-var nextLibcFd = int32(Stderr) + 1
 
 func findFreeFD() int32 {
 	var newfd int32