How older files installed by previous versions of FreeBSD are supposed to be deleted?

[Yuri]

I updated my system many times.
As a result I have many older libraries with different numbers:
/lib/libm.so.4
/lib/libm.so.5

/lib/libutil.so.5
/lib/libutil.so.7
/lib/libutil.so.8
/lib/libutil.so.9

/lib/libc.so.6
/lib/libc.so.7

How older files are normally deleted? Is there a script that finds older 
versions and deletes them? Otherwise after a while all possible versions 
will accumulate there.
I suspect the same happens under /etc with various renamed/deleted files.

Yuri