Remove reference to PostUploadActivity

Ok, so I think I'm doing this right. As I mentioned in IRC, in this commit: https://github.com/wikimedia/android-commons/commit/de5968d75e87f5e9d83aac5a3786099b4ca4693d#commons/src/main/java/org/wikimedia/commons/ShareActivity.java
you remove the file PostUploadActivity.java, but on line 20 in ShareActivity.java you make reference to PostUploadActivity: 'import org.wikimedia.commons.modifications.PostUploadActivity; '. This pull request should (I hope) remove that line.