Remove custom text codec for C strings.

Remove custom text codec for C strings.

This setting is extremely harmful, as code cannot know whether or not to expect
it. It also made the behaviour of QString::fromAscii and ::toAscii unintuitive,
and caused a lot of people to make mistakes with it.

Change-Id: I2f429fa7ef93bd75bb93a7f64c56db15b7283388
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>