iamkosmonaut, didn't say that they cleaned out the whole thing, but some stuff has gone missing over the years.
The info pertaining to the plane being scrapped is from a recent article (a week ago) in Berliner Morgenpost quoted by Aero Telegraph:
«Die Boeing 707 im Außenbereich von Tegel wird im Laufe des kommenden Jahres zerlegt und verwertet», erklärte eine Sprecherin des Museums der Morgenpost.
"The Boeing 707 at Tegel will be chopped and recycled over the course of the next year", a museum spokeswoman said.
Also confirmed by local tv news a couple of days ago. They tried to do salvage the thing, but apparently couldn't make it work.