Removing Braindump, Calligra Active, MPP import filter on Sep. 2nd

[Friedrich W. H. Kossebau]

Hi,

noone has shown up with interest to keep Braindump & Calligra Active alive and 
care for porting them to Qt5 & KF5.

So before we start with further porting & refactoring again after the freeze 
on master is melted, I would propose to remove both now in master and add them 
as some other entries to OBSOLETE.txt

Additionally I will also remove the FILTER_MPXJ_IMPORT for Plan (for importing 
MS Project files), as it is broken and noone has come around for all the time 
to fix it.

So last chance for anyone to jump in and take over any of them!

I will remove all on Tuesday, September 2nd otherwise.

Cheers
Friedrich