Remove redundant access modifiers from interface `Economy.java`

[lokka30]

The access modifiers public, etc. before every method in the interface Economy.java is redundant.

This:

@Deprecated
public double getBalance(String playerName);

becomes this:

@Deprecated
double getBalance(String playerName);

Let me know if you want me to submit a PR. Since there are already 2 PRs pending that I am excited about I don't want to cause any conflicts with them - otherwise I would've just submitted one ;)