example-configs: switch app-linux-usb-proxy to per-release branch

The devices API changes in R4.3 makes it complicated to use the same
code base for both R4.2 and R4.3. Use separate branches.

QubesOS/qubes-issues#9325

example-configs/qubes-os-master.conf

@@ -153,7 +153,6 @@ BRANCH_app_thunderbird = master
 BRANCH_app_linux_pdf_converter = master
 BRANCH_app_linux_img_converter = master
 BRANCH_app_linux_input_proxy = master
-BRANCH_app_linux_usb_proxy = master
 BRANCH_app_linux_snapd_helper = master
 BRANCH_app_shutdown_idle = master
 BRANCH_app_yubikey = master