Remove remaining vestiges of coloured text from the partition list

This commit stopped setting the text colours in the Partition, File
System and Mount Point columns to avoid hard coding text colours making
them impossible to read when using GNOME's High Contrast Inverse theme:
    ff2a6c00
    Changes post gparted-0.3.6 - code recreation from Source Forge

    * src/TreeView_Detail.cc: Removed text_color hard coding
      - Removed hard coding of Partition and Filesystem text_color
        which was based on if partition was TYPE_UNALLOCATED.
      - Removed hard coding of Mount text_color which was based
        on if partition was busy.  Lock symbol provides this indicator.
      - Closes GParted bug #413810 - Don't hardcode text colour in
        partition list

Now remove the remaining vestiges left behind.  Remove the unused color
text and mount_text_color columns from the tree model.  Also remove
setting of the column attributes which set the colour of the text in the
tree view from those unused columns in the tree model.

Unnecessary history.  Added by:
    b179990d
    show greyed-out mountpoint of unmounted partitions in the treeview
    as an improved way to identify partitions
    Bug #333027 -  Displaying unmounted partitions' default mount points
    in grey

and by commit only in CVS history:
    Bart Hakvoort <...> 2004-08-22 15:06:45
    Made text in Partition column darkgrey for unallocated. this offers
    more visual difference between partitions and unallocated space