remove jniLibs.srcDirs

[okhiroyuki]

Since Gradle 4.0, jniLibs has been removed as it is no longer needed.

refs: https://developer.android.com/studio/releases/gradle-plugin#cmake-imported-targets

error message

> Task :app:mergeDebugNativeLibs FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:mergeDebugNativeLibs'.
> A failure occurred while executing com.android.build.gradle.internal.tasks.Workers$ActionFacade
   > More than one file was found with OS independent path 'lib/arm64-v8a/libnode.so'. If you are using jniLibs and CMake IMPORTED targets, see https://developer.android.com/studio/preview/features#automatic_packaging_of_prebuilt_dependencies_used_by_cmake

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

Deprecated Gradle features were used in this build, making it incompatible with Gradle 7.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/6.5/userguide/command_line_interface.html#sec:command_line_warnings

BUILD FAILED in 20s
41 actionable tasks: 8 executed, 33 up-to-date