css-mode: Indentation inconsistent with other major modes

[Ken Hori]

css-mode.el uses its own customizable variable called
`css-indent-offset' to set the basic indentation level, which is
confusing because experienced users would normally assume setting
tab-width will have effect on the indentation level, at least for
`indent-according-to-mode'.

To remove this confusion and to be consistent with most of the
other major text/programming modes, I think `css-indent-offset'
variable should be removed to be replaced by `tab-width'.

Thanks.

-Ken