chore(windows): clean up build/packaging - drop vendored driver binaries + the LLVM-21 pin

Now that the drivers build from source in CI, remove the dead checked-in binaries and
the toolchain cruft they left behind:

- Delete packaging/windows/{pf-vdisplay,gamepad-drivers}/ (the prebuilt .dll/.inf/.cat/.cer).
  pack-host-installer.ps1 builds + signs all three drivers from the drivers/ workspace and
  nothing reads the vendored dirs anymore; stage-pf-vdisplay.ps1's -VendorDir is now a
  mandatory build-output path, not a vendored default.
- Drop the LLVM-21 pin. The vendored bindgen 0.71->0.72 bump (the shipping pack already
  builds green on the runner-default clang 22) retired the bindgen-0.71 layout-test overflow
  that needed LLVM 21.1.2, so windows-drivers.yml + provision-windows-wdk.ps1 no longer
  install/point at C:\llvm-21 (~898 MB off a fresh provision) - both driver builds now use one
  toolchain (clang 22 + bindgen 0.72).
- pack -SkipBuild on the gamepad build (build-pf-vdisplay.ps1 already builds the whole
  workspace), build-web.ps1 reaps a stale node too, deploy-dev.ps1 nefconc path + comments.
- Reword the vendored-driver references (build scripts, .iss, READMEs, the vite web-bundle
  comment) to the build-from-source reality.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/windows/build-gamepad-drivers.ps1

@@ -10,8 +10,8 @@
   `cargo build --release` builds the whole workspace (this shares wdk-sys/wdk-build + the bindgen pin with
   pf-vdisplay). Then, per driver: CLEAR the FORCE_INTEGRITY PE bit, sign the .dll, stampinf a DriverVer
   into the INF; then Inf2Cat both catalogs and sign them. Both drivers share ONE self-signed cert (or a
-  supplied DRIVER_CERT secret) + ONE exported .cer, matching the vendored gamepad-drivers/ layout that
-  install-gamepad-drivers.ps1 expects.
+  supplied DRIVER_CERT secret) + ONE exported .cer - the layout install-gamepad-drivers.ps1 consumes
+  (per-driver .inf/.cat/.dll + one shared punktfunk-driver.cer).
 
   Output (-Out): pf_dualsense.{dll,inf,cat} + pf_xusb.{dll,inf,cat} + punktfunk-driver.cer.