Platform: remove old workaround for zero monitor sizes reported by GLFW.

Back in 2019 this got hit in only one very rare case, and was fixed
in GLFW 3.3.1 in 2020. Now, with virtual DPI scaling being the default,
physical scaling is almost never used, and the chance that someone is
still on X11, suffers from this XRandR bug, explicitly uses physical
scaling and is on never-since-updated GLFW 3.3.0, is basically zero.
Yet, I'm adding an assert so it shows up something more reasonable than
a division by zero error, and in the rare chance someone actually *does*
hit that assert, they can work around it with --magnum-dpi-scaling 1 or
equivalent either via an env var or directly through code.

This reverts commit 411e349358.

src/Magnum/Platform/GlfwApplication.cpp

@@ -280,12 +280,14 @@ Vector2 GlfwApplication::dpiScalingInternal(const Implementation::GlfwDpiScaling
     #if defined(CORRADE_TARGET_UNIX) || (defined(CORRADE_TARGET_WINDOWS) && !defined(CORRADE_TARGET_WINDOWS_RT))
     GLFWmonitor* const monitor = glfwGetPrimaryMonitor();
     const GLFWvidmode* const mode = glfwGetVideoMode(monitor);
+    /* On X11, XRandR might return zero values, which would then cause a
+       division by zero here. GLFW 3.3.1+ works around that by assuming 96 DPI:
+        https://github.com/glfw/glfw/commit/7c33fb22fdd55af5417a48c8594fc495fb1f81a2
+       Subsequently, the same issue was fixed for Wayland in 3.3.3:
+        https://github.com/glfw/glfw/pull/1784 */
     Vector2i monitorSize;
     glfwGetMonitorPhysicalSize(monitor, &monitorSize.x(), &monitorSize.y());
-    if(monitorSize.isZero()) {
+    CORRADE_INTERNAL_ASSERT(!monitorSize.isZero());
-        Warning{verbose} << "Platform::GlfwApplication: the physical monitor size is zero? DPI scaling won't be used";
-        return Vector2{1.0f};
-    }
     auto dpi = Vector2{Vector2i{mode->width, mode->height}*25.4f/Vector2{monitorSize}};
     const Vector2 dpiScaling{dpi/96.0f};
     Debug{verbose} << "Platform::GlfwApplication: physical DPI scaling" << dpiScaling;