How can I remove the lock icon from Android lockscreen?

As you've already found the resource file to the arrow inside the APK, all you need to do now is to replace it with a transparent one. Root or custom recovery is required, and custom recovery is personally recommended, because there is a slight possibility that this will throw you into a bootloop, so having a backup in recovery prior to mods would be handy.

Sometimes simply extracting the APK, replacing it and repackaging it could work (this has been the case for me for simple framework mods). If custom recovery is available, trying this is fast and won't hurt.

The safer way is to decompile the package while keeping its signature. Virtuous Ten Studio is a friendly way to get your hands on this without getting lost in the command lines of APKTool - the site also has simple tutorials.

After modification and repackaging, overwrite the original one and set correct permissions (usually rw-r--r-- (0644)). Reboot for changes to take effect.

And at last - do not simply delete the file, or whenever the file is used, the app will error out, possibly leaving you with endless Force Close dialogs.