[Fuchsia] Remove fuchsia_target_api_level from .gn

We get this automatically from the //build checkout now

Bug: chromium:1432399
Change-Id: I223d7c5448244ed62821207068f979555617da57
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/318560
Auto-Submit: Chong Gu <chonggu@google.com>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Zijie He <zijiehe@chromium.org>
Cr-Commit-Position: refs/heads/main@{#40686}

.gn

@@ -74,5 +74,4 @@ default_args = {
   # Chromium fix resolves the problem.
   fuchsia_sdk_readelf_exec =
       "//third_party/llvm-build/Release+Asserts/bin/llvm-readelf"
-  fuchsia_target_api_level = 11
 }