livepatheffect-add: Remove this deprecated Gallery

as per discussion on chat.inkscape.org:

>>>
dboles 1:58 PM
The Live Path Effect has a preference that enables this deprecated
selector, by revealing the button to open it. Since this is deprecated,
can I assume we need not prep it for GTK4, and can instead just remove
it during the migration? It uses a bunch of EventBoxes, which are not in
GTK4, and may or may not be a pain to replace / rewrite input handling
for.
it is marked as deprecated in the preference and the LPE code, i mean
the non-deprecated parts of the LPE dialog are already scary enough to
understand and maintain, so it would be nice if we don't have to migrate
this bit...
said dialog used EventBoxes instead of Buttons in a bunch of places, for
no apparent reason, so those are now just made into Buttons... and the
only EventBox left is to get expander behaviour on the entire width of
the header of each individual effect shown (hopefully easy enough to
replace in GTK4)

mikekob 2:33 PM
I think dropping that gallery would be OK.

jabiertxof Admin 🤖 2:54 PM
is ok remove it
    👍 1
>>>

Also, remove unused boolean member, & exit early in a bigly-indented fn.
And in the remaining UI file remove now-redundant single-child GtkBoxes.