Compiler warnings for unused symbols in `GA_SettingsInspector.cs` and `GA_SignUp.cs`

[Grimeh]

There are several unused symbols in GA_SettingsInspector.cs and GA_SignUp.cs that generate compiler warnings.

btnSize in GA_SettingsInspector:1491:

GA-SDK-UNITY/Editor/GA_SettingsInspector.cs

Line 1491 in 75366dc

const int btnSize = 12;

And _signUpInProgress, _createGameInProgress, _googlePlayPublicKey, _selectedPlatform, _selectedOrganization, _selectedStudio in GA_SignUp:38-43:

GA-SDK-UNITY/Editor/GA_SignUp.cs

Lines 38 to 43 in 75366dc

	private bool _signUpInProgress = false;
	private bool _createGameInProgress = false;
	private string _googlePlayPublicKey = "";
	private RuntimePlatform _selectedPlatform;
	private int _selectedOrganization;
	private int _selectedStudio;

The last 3 don't generate "assigned but not used" warnings, but they're all unused and seem related so it makes sense to remove them all imo.