How to remove the android OS version from the DHCP request?
It's hard-coded in Java DhcpClient:

...
    private String getVendorId() {
        if (testOverrideVendorId != null) return testOverrideVendorId;
        return "android-dhcp-" + Build.VERSION.RELEASE;
    }
...


So the only way is to rebuild your ROM with modified source code. Or use a different DHCP client from command-line. For reference: Connecting to WiFi via ADB Shell.