Remove the context menu feature

This feature sadly never garnered enough implementation interest, being
only implemented in Gecko. For some time it looked as though Blink was
also interested in implementing, but they recently made the decision to
cease implementation and remove all of their code for it, per
https://bugs.chromium.org/p/chromium/issues/detail?id=87553&desc=2#c64.

Closes #2730.

Author: domenic

.htaccess

@@ -45,6 +45,10 @@ Redirect 301 /multipage/tabular-data.html /multipage/tables.html
 # Intentionally deleted previously-generated filenames for /multipage/:
 Redirect 410 /multipage/section-sql.html
 
+# Images for now-deleted features
+Redirect 410 /images/contextmenu-collapsed.png
+Redirect 410 /images/contextmenu-expanded.png
+
 # Images converted to svg:
 Redirect 301 /images/content-venn.png /images/content-venn.svg
 Redirect 301 /images/parsing-model-overview.png /images/parsing-model-overview.svg