Remove .gitmodules and references to submodules in SETUP.md

While setting up a clone of this repo I noticed that .gitmodules was empty
and that no submodules were being used. However, SETUP.md still referenced them.

This commit cleans that up. The empty .gitmodules is deleted and the references
to modules in SETUP are deleted too.

SETUP.md

@@ -32,7 +32,7 @@ The next steps will assume you have a GitHub account and that you will get the c
 
 * In a web browser, go to https://github.com/nextcloud/android, and click the 'Fork' button near the top right corner.
 * Open a terminal and go on with the next steps in it.
-* Clone your forked repository: ```git clone --recursive https://github.com/YOURGITHUBNAME/android.git```.
+* Clone your forked repository: ```git clone https://github.com/YOURGITHUBNAME/android.git```.
 * Move to the project folder with ```cd android```.
 * Pull any changes from your remote branch 'master': ```git pull origin master```
 * Make official Nextcloud repo known as upstream: ```git remote add upstream https://github.com/nextcloud/android.git```
@@ -49,7 +49,6 @@ We recommend to use the last version available in the stable channel of Android
 
 To set up the project in Android Studio follow the next steps:
 
-* Make sure you have called ```git submodule update``` whenever you switched branches
 * Open Android Studio and select 'Import Project (Eclipse ADT, Gradle, etc)'. Browse through your file system to the folder 'android' where the project is located. Android Studio will then create the '.iml' files it needs. If you ever close the project but the files are still there, you just select 'Open Project…'. The file chooser will show an Android face as the folder icon, which you can select to reopen the project.
 * Android Studio will try to build the project directly after importing it. To build it manually, follow the menu path 'Build'/'Make Project', or just click the 'Play' button in the toolbar to build and run it in a mobile device or an emulator. The resulting APK file will be saved in the 'build/outputs/apk/' subdirectory in the project folder.
 * Setup Android Studio editor configurtation for the project: ```Settings``` → ```Editor``` → ```Code Style``` → ```Scheme: Project``` and ```Enable EditorConfig support```
@@ -60,7 +59,6 @@ To set up the project in Android Studio follow the next steps:
 [Gradle][7] is the build system used by Android Studio to manage the building operations on Android apps. You do not need to install Gradle in your system, and Google recommends not to do it, but instead trusting on the [Gradle wrapper][8] included in the project.
 
 * Open a terminal and go to the 'android' directory that contains the repository.
-* Make sure you have called ```git submodule update``` whenever you switched branches
 * Run the 'clean' and 'build' tasks using the Gradle wrapper provided
     - Windows: ```gradlew.bat clean build```
     - Mac OS/Linux: ```./gradlew clean build```