Currently if there is no selection mode enabled the operator is disabled and greyed
out in the menu. The shortcut doesn't do anything, which can be unexpected and jarring.
With this PR the operator will work when no selection mode is enabled and in that case
fill the entire mesh using the Weight from the active tool.
This is then consistent with vertex paint mode and the "Set Vertex Color" operator.
So now remove the overly-strict `mask_paint_poll` poll (and replace with
more appropriate `weight_paint_mode_poll`).
NOTE: `mask_paint_poll` is now unused, could be removed
Pull Request: https://projects.blender.org/blender/blender/pulls/112413
0b1d2d63fe