sign in button had corners that did not match its background (#552)

## Summary
Sign In button had dark corners on light mode although it had rounded
corners. The two did not look good together.

## Validation
ran the app in the simulator

## Issue
Fixes #551 
Related #483

BeeSwift/SignInViewController.swift

@@ -81,7 +81,6 @@ class SignInViewController : UIViewController, UITextFieldDelegate {
         scrollView.addSubview(self.signInButton)
         self.signInButton.isHidden = true
         self.signInButton.setTitle("Sign In", for: UIControl.State())
-        self.signInButton.backgroundColor = UIColor.Beeminder.gray
         self.signInButton.titleLabel?.font = UIFont.beeminder.defaultFontPlain.withSize(20)
         self.signInButton.titleLabel?.textColor = UIColor.white
         self.signInButton.addTarget(self, action: #selector(SignInViewController.signInButtonPressed), for: UIControl.Event.touchUpInside)