This came with rBf8a0e102cf5e: Weightpaint Gradient tool: expose falloff to the UI.

The panel was meant specifically for the gradient tool, but since it was
given the ".weighpaint" context, it would also draw as part of generic
header toolsettings drawing.

Now remove this context on purpose and only draw this specifically from
the gradient tools ToolDef.

Also add an extra check in poll to make sure we only draw this for the
right tool.