ramips: mt7530: remove redundant global attrs for port mirroring

ramips: mt7530: remove redundant global attrs for port mirroring

Global attributes enable_mirror_tx/enable_mirror_rx depend on runtime
value of another global attribute mirror_source_port which just resides
in the memory

The same functionality can be achieved by directly setting port
attribute of the same names.  E.g. the following two groups of commands
achieve the same thing

	swconfig dev switch0 set mirror_source_port 3
	swconfig dev switch0 set enable_mirror_tx 1
	swconfig dev switch0 set mirror_source_port 4
	swconfig dev switch0 set enable_mirror_tx 1

	swconfig dev switch0 port 3 set enable_mirror_tx 1
	swconfig dev switch0 port 4 set enable_mirror_tx 1

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>