ath25: remove target specific /etc/config/system

The system config file shipped by ath25 is now equivalent to the generic one
in base files, so drop the target specific copy.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/ath25/base-files/etc/config/system

@@ -1,9 +0,0 @@
-config system
-	option hostname OpenWrt
-	option timezone UTC
-
-config timeserver ntp
-	list server	0.openwrt.pool.ntp.org
-	list server	1.openwrt.pool.ntp.org
-	list server	2.openwrt.pool.ntp.org
-	list server	3.openwrt.pool.ntp.org