Revert "Fuchsia: Relocate fidl_library.gni to build/config/fuchsia."

This reverts commit 0d16a926bfb155fcc2272bf1f906cb092ed45b1a.

Reason for revert:
This broke the analyze step on fuchsia cq trybots, e.g.
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/fuchsia_x64/36927:

ERROR at //webrunner/BUILD.gn:8:1: Can't load input file.
import("//third_party/fuchsia-sdk/fidl_library.gni")
^--------------------------------------------------
Unable to load:
  /b/swarming/w/ir/cache/builder/src/third_party/fuchsia-sdk/fidl_library.gni
I also checked in the secondary tree for:
  /b/swarming/w/ir/cache/builder/src/build/secondary/third_party/fuchsia-sdk/fidl_library.gni
See //BUILD.gn:757:7: which caused the file to be included.
      "//webrunner",
      ^------------
GN gen failed: 1

Original change's description:
> Fuchsia: Relocate fidl_library.gni to build/config/fuchsia.
> 
> fidl_library.gni will be used for targets defined outside "third_party",
> so promoting the GN template to a higher level makes sense.
> 
> Change-Id: I723419be11c450ff78729320bb04f908aa486f22
> Reviewed-on: https://chromium-review.googlesource.com/1089711
> Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#565209}

TBR=dcheng@chromium.org,kmarshall@chromium.org,sergeyu@chromium.org

Change-Id: Ib91288a81b7f9e71e480e3fe95d8499af28fa6c6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1090650
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#565211}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b437a8d206476ec1e6ca6074440bd004b046d97a