gluon-core: add support for removing sysctl settings

package/gluon-core/luasrc/usr/lib/lua/gluon/sysctl.lua

@@ -4,5 +4,9 @@ local util = require 'gluon.util'
 module 'gluon.sysctl'
 
 function set(name, value)
-	util.replace_prefix('/etc/sysctl.conf', name .. '=', name .. '=' .. value .. '\n')
+	local new
+	if value then
+		new = name .. '=' .. value .. '\n'
+	end
+	util.replace_prefix('/etc/sysctl.conf', name .. '=', new)
 end