docs: drop the WebGL/GPU renderer claim from the stealth profile section

The profile section promises that each profile keeps "the WebGL/GPU renderer
internally consistent" and that Windows profiles report ANGLE Direct3D11 and
macOS profiles ANGLE Metal. That was true when the sentence was written in
#288, against the WebGL shim that existed then.

29d0bce ("render: expose WebGL as unavailable without backend") made
`getContext('webgl')`, `'experimental-webgl'` and `'webgl2'` return `null`,
deliberately, with an in-source comment explaining why. So there is no
renderer string for a page to read, and nothing for the profile to be
consistent with.

Checked on main (f449e6f) and on the v0.2.0 stealth binary:

    obscura fetch "data:text/html,<p>x" \
      --eval "String(document.createElement('canvas').getContext('webgl'))"
    null

Doc-only. The `gpu` and `gpuVendor` fields are still assigned into the
fingerprint object in bootstrap.js and are not read anywhere; whether to
remove them or to wire them back up when WebGL lands is a separate call, so
this change does not touch them.