Remove the deprecated "All rights reserved" phrase from all copyright notices

[delgh1]

"The requirement to add a notice became obsolete and essentially
deprecated on August 23, 2000, as every country that was a member of
the Buenos Aires Convention (which is the only copyright treaty
requiring this notice to be used) is also a member of the Berne
Convention which requires protection be granted without any formality
of notice of copyright."

taken from debian-legal mailing list

The next reply of the email says,

> 5) Is it *advisable* to use such a terminology in a copyright statement 
> in a file licensed under the GPL?

No.

Since all copyright notices in mybb are after 2002, I suggest removing this phrase from all copyright notices.

[Stefan-MyBB]

It makes little sense to touch every single file in a minor update.