Since 18.06, OpenWrt no longer provide image for tl-wr703n, you have to build it on your own.
If your device is supported by latest OpenWrt, you might want to download Stable Release or Development Snapshots from here.

Prepare to build

Install Build system
https://openwrt.org/docs/guide-developer/build-system/install-buildsystem

Clone source code and install feeds

git clone --branch=openwrt-18.06 https://github.com/openwrt/openwrt.git
cd openwrt
./scripts/feeds update -a
./scripts/feeds install -a

This will build the latest Snapshot. If you want stable release. git checkout [TAG].

Add tl-wr703n-v1 back

After I took some look at the source code.
Edit target/linux/ar71xx/image/generic-tp-link.mk.
Add the tl-wr703n-v1 define above tl-wr710n-v1. Note the TPLINK_HWID is different with tl-wr710n.

define Device/tl-wr703n-v1
  $(Device/tplink-8mlzma)
  DEVICE_TITLE := TP-LINK TL-WR703N v1
  DEVICE_PACKAGES := kmod-usb-core kmod-usb2
  BOARDNAME := TL-WR703N
  DEVICE_PROFILE := TLWR703
  TPLINK_HWID := 0x07030101
  CONSOLE := ttyATH0,115200
  IMAGE/factory.bin := append-rootfs | mktplinkfw factory -C US
endef
TARGET_DEVICES += tl-wr703n-v1

Configure build

Use this config.seed (modify link if you are building for stable release)

wget https://downloads.openwrt.org/snapshots/targets/ar71xx/generic/config.seed -O config.seed
rm -rf .config*
mv config.seed .config

Add the following two lines to .config

CONFIG_TARGET_DEVICE_ar71xx_generic_DEVICE_tl-wr703n-v1=y
CONFIG_TARGET_DEVICE_PACKAGES_ar71xx_generic_DEVICE_tl-wr703n-v1=""

You should see TP-LINK TLWR703N v1 appeared in Target Devices.

make defconfig
make menuconfig

Start to build

This will take minutes to hours.

time make -j20 V=s | tee build.log

Flash new image

Upload image to router (OpenWrt) and upgrade.

scp bin/targets/ar71xx/generic/openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-sysupgrade.bin [email protected]:/tmp/
ssh [email protected]
sysupgrade -i /tmp/openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-sysupgrade.bin