Remove support for Maruku Markdown processor

Maruku has been marked as obsolete by its maintainer [1], and support
was removed from Tilt 2.5.0. [2]

I considered deprecating it in Nesta 0.19.0 and removing it in 0.20.0,
but as there are no significant new features in 0.19.0 I felt it was
simpler all round to just remove it.

[1] bhollis/maruku@4d88d0e
[2] https://github.com/jeremyevans/tilt/blob/master/CHANGELOG.md#250-2024-12-20

CHANGELOG.md

@@ -2,6 +2,15 @@
 
 ## 0.19.0 (Unreleased)
 
+* Remove support for the Maruku Markdown processor.
+
+  Maruku is considered obsolete by its maintainer, and is no longer used by
+  Tilt (as of Tilt 2.5.0). As Nesta uses renders Markdown via Tilt, there's no
+  need for us to continue loading Maruku via Tilt. It's recommended that you
+  update your site to use kramdown.
+
+  (Suggested by Brad Weslake)
+
 * Fix whitespace in version.rb when generating new plugins. (Graham Ashton)
 
 ## 0.18.0 (19 November 2024)

lib/nesta/models/file_model.rb

@@ -5,7 +5,6 @@ def register_template_handler(class_name, *extensions)
   # safely ignore these load errors.
 end
 
-register_template_handler :MarukuTemplate, 'mdown', 'md'
 register_template_handler :KramdownTemplate, 'mdown', 'md'
 register_template_handler :RDiscountTemplate, 'mdown', 'md'
 register_template_handler :RedcarpetTemplate, 'mdown', 'md'