chore: drop the dead help window's .ui with the window it built

`help_window.cpp`'s paged window went with the manual (c1460a6), leaving `src/ui/help.ui` a builder
file that nothing loads and nothing references: its only mention was the bundle manifest beside the
icons. Removed both. That is also the reading `test-ui` now reports -- 13 files, 13 ok -- and no
document stated the old count, so nothing else had to move.

Verified on the integrated tree: clean build --clean-first, 0 errors and exactly the three documented
warnings (the two MathNode missing-initializers and the icons' 12352-byte overlength string); test-ui
13 ok, 0 failed; test-help renders all 15 pages; and two headless probe runs report 0 Gtk-CRITICAL:

    [help] topics=14 index=1 figures=3 ... shortcuts: app=14 plot=14 tables=28 ui=28 ui_accel=21
           ui_icon=7 ui_bare=0 ... closed ok
    [helpui] auto_started=1 visible=1 steps=5 step=0 (tour_seen already set)
    [helpui] contextual on: hints=8 ids={events plot status cursors measures trigger signals window}
    [helpui] main view: rows=7 help_row=1

and the trigger pane's compact Arm/Disarm -- the feature from 620473a -- still reads
`visible=1 sensitive=1 width=51 label=Disarm` with `panel_width=-1` while the pane is locked.

src/ui/udpscope-ui.gresource.xml

@@ -12,7 +12,6 @@
   <gresource prefix="/org/udpscope/ui">
     <file preprocess="xml-stripblanks">shortcuts.ui</file>
     <file preprocess="xml-stripblanks">preferences.ui</file>
-    <file preprocess="xml-stripblanks">help.ui</file>
     <file preprocess="xml-stripblanks">source-form.ui</file>
     <file preprocess="xml-stripblanks">source-chooser.ui</file>
     <file preprocess="xml-stripblanks">loading.ui</file>