[openwrt] Remove automatic timezone

[nemesifier]

At the moment the following configuration dictionary:

OpenWrt({'general': {'hostname': 'test'}})

generates the following output:

package system

config system 'system'
	option hostname 'test'
	option timezone 'UTC'
	option zonename 'UTC'

Since timezone on OpenWRT defaults to UTC even if not specified, there's no need to automatically add timezone and zonename if the NetJSON timezone is not specified.