Docs: Migrate gvariant-*.svg licenses to comments

Remove the overrides in `.reuse/dep5`, and instead store the license
information in a comment near the top of each file.

docs/reference/glib/gvariant-byte-boundaries.svg

@@ -1,5 +1,10 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <!-- Created with Inkscape (http://www.inkscape.org/) -->
+<!--
+SPDX-License-Identifier: CC-BY-SA-3.0
+
+SPDX-FileCopyrightText: 2022 Philip Withnall
+-->
 
 <svg
    width="43.395473mm"

docs/reference/glib/gvariant-integer-and-string-structure.svg

@@ -1,5 +1,10 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <!-- Created with Inkscape (http://www.inkscape.org/) -->
+<!--
+SPDX-License-Identifier: CC-BY-SA-3.0
+
+SPDX-FileCopyrightText: 2022 Philip Withnall
+-->
 
 <svg
    width="116.87785mm"

docs/reference/glib/gvariant-integer-array.svg

@@ -1,5 +1,10 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <!-- Created with Inkscape (http://www.inkscape.org/) -->
+<!--
+SPDX-License-Identifier: CC-BY-SA-3.0
+
+SPDX-FileCopyrightText: 2022 Philip Withnall
+-->
 
 <svg
    width="104.43805mm"

docs/reference/glib/gvariant-string-array.svg

@@ -1,5 +1,10 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <!-- Created with Inkscape (http://www.inkscape.org/) -->
+<!--
+SPDX-License-Identifier: CC-BY-SA-3.0
+
+SPDX-FileCopyrightText: 2022 Philip Withnall
+-->
 
 <svg
    width="149.22821mm"