[Fuchsia] remove build/fuchsia (#839)

After flutter/engine#51072, this folder is
obsolete and can be removed.

The change is locally tested.

Bug: http://b/40935282

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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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---------

Co-authored-by: Jason Simmons <jason-simmons@users.noreply.github.com>

build/config/compiler/BUILD.gn

@@ -17,7 +17,6 @@ if (is_win) {
 import("//build/config/c++/c++.gni")
 import("//build/config/profiler.gni")
 import("//build/config/sanitizers/sanitizers.gni")
-import("//build/fuchsia/config.gni")
 import("//build/toolchain/ccache.gni")
 import("//build/toolchain/clang.gni")
 import("//build/toolchain/toolchain.gni")
@@ -427,9 +426,7 @@ config("compiler") {
   # Fuchsia-specific flags setup.
   # -----------------------------
   if (is_fuchsia) {
-    if (fuchsia_target_api_level != -1) {
-      cflags += [ "-ffuchsia-api-level=${fuchsia_target_api_level}" ]
-    }
+    configs = ["//flutter/tools/fuchsia/gn-sdk/src/config:compiler"]
   }
 
   asmflags = cflags
@@ -582,6 +579,11 @@ config("runtime_library") {
       libs += [ "gcc" ]
     }
   }
+
+  # Fuchsia standard library setup.
+  if (is_fuchsia) {
+    configs = ["//flutter/tools/fuchsia/gn-sdk/src/config:runtime_library"]
+  }
 }
 
 # default_warning_flags collects all warning flags that are used by default.