Remove outdated comment about ped_partition_is_busy() for DMRaid devices

Comment is outdated after GParted stopped using ped_partition_is_busy()
for busy partition detection of normal and logical partitions since
commit:

    420299206323fb49da5ba099b9f51cba95e58603
    Fix false busy detection of unusual case with Linux Software RAID (#712533)
Remove unused member variable GParted_Core::buf
Remove set_proper_filesystem() method

Prior to commit:

    1f3b11748e99c0b56fed35a3fc77b2da5dc3072e
    Remove GParted_Core::p_filesystem (#683149)

set_proper_filesystems() used to set GParted_Core::p_filesystem member
variable to one of the FileSystem objects, but that was just treating it
like a local variable.  After the commit local variables named
p_filesystem were used where required and set_proper_filesystem() became
a function which did nothing other than call get_filesystem_object().

Now remove set_proper_filesystem() altogether and use
get_filesystem_object() in its place.
Initialise file system objects only once

The code used to unnecessarily destroy and re-create the file system
objects on every scan for file system support tools.

Instead only create the file system objects once and just call each
object's get_filesystem_support() method on each rescan.