libwacom_has_eraser tells us if this non-eraser tool has an
associated eraser (same serial, different tool id)
libwacom_is_eraser tells us if the current tool is an eraser device.
Now that we always have the stylus for the tool in proximity (see the
previous commit), switch this back to the has_eraser function and
rename it in the process so it's more obviously not the seemingly
obvious interpretation.
This reverts commit 9bd0b94527.
Similar to CcWacomDevice vs GsdWacomDevice, CcWacomTool is meant to
replace the GsdWacomStylus object. There is a substantial difference
between these two objects, CcWacomTool offers constructor methods,
it expects the caller to maintain lifetime otherwise. while
GsdWacomStylus objects creation was rather fixed (GsdWacomDevice
created all possible styli on initialization). This latter model
doesn't help us use all the possibilities wrt wayland configurability.
