remove magick variable, use convert for all platform

combine.lua

@@ -20,19 +20,15 @@ for i = 1, len do
     origin[i] = file_dir .. "/" .. data[i] .. " "
 end
 -- 从 nginx 变量取
-local magick = ngx.var.magick
 local convert = ngx.var.convert
 
-if not magick then
 if not convert then
     ngx.status = 500
     ngx.log(ngx.ERR, "ImageMagick NOT FOUND.")
     return
 end
-    magick = ""
-end
 
-local cmd = magick .. " " .. convert .. " -append " .. table.concat(origin, " ") .. " " .. combined  
+local cmd = convert .. " -append " .. table.concat(origin, " ") .. " " .. combined
 ngx.log(ngx.INFO, 'CMD -> ', cmd)
 local result, _, code = os.execute(cmd)
 if result and code == 0 then