text-view: Do not double-allocate borders

text-view: Do not double-allocate borders

Reverts part of Commit 25b67af3

The 'width' part of the commit is the cause of #628: requisition->width
is first set to priv->layout->width, which already includes
priv->left_border + priv->right_border. It's a bit labyrinthine, but
essentially:

*  layout->width is set in update_layout_size() (gtktextlayout.c line 992)
   as the maximum line width, and
*  the line width is set to display->width in gtk_text_layout_real_wrap()
   (gtktextlayout.c line 1183), and
*  display->width is set to text_pixel_width + h_margin + h_padding in
   gtk_text_layout_get_line_display() (gtktextlayout.c line 2584), and
*  h_margin + h_padding is the same as priv->left_border +
   priv->right_border.

Adding it again leads to an increase in the size-request, which
results in wider lines; rinse and repeat.