wacom: do not bail out if the tablet doesn't have an eraser

Tablets have not always an eraser (most of the generic tablets like Huion,
UC-Logic, etc... don't). We should not reject such tablets.

Commit 54849a9e (wacom: Only the stylus and eraser tools need to exist)
mentioned that we were not sure about eraser, and I think we should not
assume one either.

To do so, we simply ignore the eraser xinput node and rely on
libwacom to actually provide the eraser information.

If the stylus does not have the eraser tip, we may fall in the
LAYOUT_OTHER case. We have a picture of a generic Wacom pen with an
eraser, and the leaders linking the widget to the picture are scrambled.

To prevent that, gray out the eraser pressure slider so that we do not
break the layout.

https://bugzilla.gnome.org/show_bug.cgi?id=746117