chore: drop the math icon, which lost its last user with the toggles

`udpscope-math-symbolic` was the toolbar's math-pane toggle, and that toggle is gone. Nothing
else names it: the census after the change is **five** action icons
(`udpscope-{trigger,cursors,measure,events,resize-rows}-symbolic.svg`) plus the two application
icons, seven bundled.

`udpscope-measure-symbolic` **stays**, and the plan's premise ("both lost their last user") is
wrong for it: `src/ui/preferences.ui` carries it as the Plot preferences page's icon -- a live
user, unrelated to the toggle that introduced it. Deleting it would have shipped a page whose
icon silently fails to resolve (GTK substitutes a placeholder and says nothing), so it stays and
the doc's census names it as the Preferences icon rather than as a toolbar one.

The warning baseline is unchanged: **four**, because the icons' generated GResource blob is
**8613 bytes** against C99's 4095-byte string-literal limit with the five action icons and the
two app icons -- removing one file did not put it back under, and neither figure moved on a
clean build. Measured by touching the two `*.gresource.xml` files and rebuilding, which
regenerates both blobs without one.

Verified: `make test-ui` 14 files; a run with the Preferences dialog opened resolves every icon
(`toolbar icons:` all `(ok)`, zero `Gtk-CRITICAL`, no "not present").

src/icons/scalable/actions/udpscope-math-symbolic.svg

@@ -1,13 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!-- SPDX-License-Identifier: MIT -->
-<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
-<!-- A sigma: the one pane that computes rather than reads. Four filled subpaths, all
-     wound clockwise so the default nonzero rule unions them where they overlap; the
-     diagonals run from the bars' left ends to a vertex right of centre, which is the
-     way the glyph actually points. -->
-<svg height="16px" width="16px" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg">
-  <path fill="#2e3436" d="M3 2 H13 V4 H3 Z
-                          M3 12 H13 V14 H3 Z
-                          M4.64 2.23 L10.64 7.23 L9.36 8.77 L3.36 3.77 Z
-                          M10.64 8.77 L4.64 13.77 L3.36 12.23 L9.36 7.23 Z"/>
-</svg>

src/icons/udpscope.gresource.xml

@@ -20,7 +20,6 @@
     <file preprocess="xml-stripblanks">scalable/actions/udpscope-trigger-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/udpscope-cursors-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/udpscope-measure-symbolic.svg</file>
-    <file preprocess="xml-stripblanks">scalable/actions/udpscope-math-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/udpscope-events-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/actions/udpscope-resize-rows-symbolic.svg</file>
     <file preprocess="xml-stripblanks">scalable/apps/org.udpscope.app.svg</file>