Remove deprecated jni.srcDirs from ReactAndroid build file

Summary:
As the title says, android.sourceSets.main.jni is deprecated in AGP. We should not be accessing it. Here we were setting it to the empty array (the default value).

It will cause the build to break in a future AGP bump, hence I'm removing it.

Changelog:
[Internal] [Changed] - Remove deprecated jni.srcDirs from ReactAndroid build file

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: cipolleschi

Differential Revision: D38655857

fbshipit-source-id: 2eb6897964554da462bde58937a6de708bc047dc

ReactAndroid/build.gradle

@@ -335,7 +335,6 @@ android {
     preBuild.dependsOn("generateCodegenArtifactsFromSchema")
 
     sourceSets.main {
-        jni.srcDirs = []
         res.srcDirs = ["src/main/res/devsupport", "src/main/res/shell", "src/main/res/views/modal", "src/main/res/views/uimanager"]
         java {
             srcDirs = ["src/main/java", "src/main/libraries/soloader/java", "src/main/jni/first-party/fb/jni/java"]