Drop splash screen

Summary:
KDevelop 5 starts significantly faster with all the changes in the
plugin loading mechanisms. I've also been able to reduce start up time
of the welcome page plugin significantly lately. Still having a splash
screen seems old-fashioned, plus adds even more delay to the startup.

Thus finally want to kill it.

More reasons for removing the splash:

• Delays startup unnecessarily
• The splash screen is still broken in some configurations: Bug: ~10 px border on the right for instance (not going to fix it)
• It's 2016, splash screens are kind of out of fashion.

See discussion at:

https://mail.kde.org/pipermail/kdevelop-devel/2016-February/053120.html

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D1912