Remove the BUCK file from rn-tester. (#37140)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/37140

This is effectively unused so I'm removing it entirely.

Changelog:
[Internal] [Changed] - Remove the BUCK file from rn-tester

Reviewed By: cipolleschi

Differential Revision: D45390818

fbshipit-source-id: 791b6a589d5ca1b56fa720e232efb0c7fab0d9c3

packages/rn-tester/android/app/_BUCK

@@ -1,39 +0,0 @@
-load("//tools/build_defs/oss:rn_defs.bzl", "KEYSTORE_TARGET", "react_native_dep", "react_native_target", "rn_android_binary", "rn_android_library", "rn_android_resource")
-
-rn_android_binary(
-    name = "app",
-    keystore = KEYSTORE_TARGET,
-    manifest = "src/main/AndroidManifest.xml",
-    deps = [
-        ":rntester-lib",
-    ],
-)
-
-rn_android_library(
-    name = "rntester-lib",
-    srcs = glob(["src/main/java/**/*.java"]),
-    deps = [
-        ":res",
-        react_native_dep("libraries/soloader/java/com/facebook/soloader:soloader"),
-        react_native_dep("third-party/android/androidx:annotation"),
-        react_native_dep("third-party/android/androidx:appcompat"),
-        react_native_dep("third-party/android/androidx:core"),
-        react_native_dep("third-party/android/androidx:fragment"),
-        react_native_dep("third-party/android/androidx:legacy-support-core-utils"),
-        react_native_dep("third-party/java/jsr-305:jsr-305"),
-        react_native_target("java/com/facebook/react:react"),
-        react_native_target("java/com/facebook/react/common:build_config"),
-        react_native_target("java/com/facebook/react/modules/core:core"),
-        react_native_target("java/com/facebook/react/views/text:text"),
-        react_native_target("java/com/facebook/react/shell:shell"),
-        react_native_target("jni/prebuilt:android-jsc"),
-        # .so files are prebuilt by Gradle with `./gradlew :ReactAndroid:packageReactNdkLibsForBuck`
-        react_native_target("jni/prebuilt:reactnative-libs"),
-    ],
-)
-
-rn_android_resource(
-    name = "res",
-    package = "com.facebook.react.uiapp",
-    res = "src/main/res",
-)