In docx a colour value is represented as a 6-digit hex RGB value, or
alternatively the word "auto" to represent automatic colour.

 - Add support for reading the value "auto" as COL_AUTO.  Previously
   this would be read as if it were a hex value, stopping at the
   letter 'u' which is not a valid hex digit, resulting in the colour
   0x00000A - a very dark blue, which looks close enough to black that
   it went unnoticed for a long time :-)

 - Remove code which tried to handle this wrong 0x00000A value,
   including the constant OOXML_COLOR_AUTO, as it is no longer needed
   and will cause surprises for anyone who really wanted this exact
   shade of dark blue

 - Fix unit tests that were checking for 0x00000A

Change-Id: I6000070341931147ff9341ad6281cd3b53c02b46
Reviewed-on: https://gerrit.libreoffice.org/50995Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>