iperf: drop PKG_BUILD_DIR override

No longer necessary since the removal of build variants

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/network/utils/iperf/Makefile

@@ -11,7 +11,6 @@ PKG_NAME:=iperf
 PKG_VERSION:=2.0.9
 PKG_RELEASE:=1
 
-PKG_BUILD_DIR:=$(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-$(PKG_VERSION)
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=@SF/iperf2
 PKG_MD5SUM:=1bb3a1d98b1973aee6e8f171933c0f61