[roll] Roll fuchsia [bazel] Remove //build/bazel/platforms:fuchsia_platform

This platform is not currently used, and if it is accidentally used,
such as in a transition, the `bazel-out/` subdirectory is like
"fuchsia_platform-opt" instead of "fuchsia_platform_${target_cpu}-opt"
for the actual CPU-specific platform it aliases. This can result in
building the same targets multiple times.

For developers, we already have a Bazel config alias "fuchsia_platform"
that uses the actual CPU-specific platform. Thus,
`fx bazel build --config=fuchsia_platform <fuchsia_target>` and
``fx bazel build --config=fuchsia_platform_${target_cpu} <fuchsia_target>`
both use `//bazel-out/fuchsia_platform_${target_cpu}-opt`.

Original-Original-Bug: 520546777, 502227168
Test: fx build //:default //build/bazel/rules/tests
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1639956
Original-Original-Revision: dbe473b5e3d318941229518d90e0e754ebc8503c
GitOrigin-RevId: c10c904efbcadb5a2188cb0f215322d6f2307b14
Change-Id: Ie302a96ae7fd4f0c3097281a12cc7162d4613bb2
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1644148
Cr-Commit-Position: refs/heads/main@{#194372}