x11: Fix event.is_action() for release of modifier keys
x11: Fix event.is_action() for release of modifier keys

The bug was that the release events for these also had the modifier state set, so the event comparison
failed.

Fixes #5901

(cherry picked from commit 6fcf2b2bd87e16c9cfc55f3c1293797c24124e85)

pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1