Comment out FileUnlink of excess segments during mdtruncate().

Comment out FileUnlink of excess segments during mdtruncate().
This is unsafe in 6.5 because other backends may be able to access the file
before noticing the shared cache inval message that tells 'em to re-open
the file.  We have fixed this for 6.6 but the changes seem too risky to
back-patch for 6.5.2.
Also, back-patch Tatsuo's change to prevent creation of files during
mdopen().