[Fuchsia] Remove reserved table field

This removes the only occurrence of a reserved FIDL field in chromium,
in application_config.fidl. Explicitly marking reserved fields has
been optional since https://fxrev.dev/1015564, and soon will be removed
from the FIDL language.

Bug: https://fxbug.dev/330609159
Change-Id: I8911d23a9b3cf75745091e17b298833e62d3420e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5421651


Auto-Submit: Mitchell Kember <mkember@google.com>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Owners-Override: Srinivas Sista <srinivassista@chromium.org>
Commit-Queue: Mitchell Kember <mkember@google.com>
Reviewed-by: Zijie He <zijiehe@google.com>
Cr-Commit-Position: refs/heads/main@{#1283371}