misc: remove extra_data function

doc/function/extra_data.txt

@@ -1,27 +0,0 @@
-Function: extra_data
-
-DOC_MSE_VERSION: since 2.2.0
-
---Usage--
-> extra_data("field_name")
-
-Access the value of extra card fields.
-
-Due to lazy initialization of stylesheet specific Extra Card Fields they are not exposed directly from the card using `card.field_name` syntax.
-As a workaround this function returns the Value of the specified field name for the active card using an access pattern that supports late resolution of the value for the active stylesheet.
-
---Parameters--
-! Parameter	Type	Description
-| @input@	[[type:string]]	Field name, with or without underscores.
-
---Examples--
-
->extra card field:
->	...
->	name: my first extra field
->
->extra card field:
->	...
->	name: my second extra field
->	script:
->>		space_to_comma(extra_data("my first extra field")) + "!"

doc/function/index.txt

@@ -65,8 +65,7 @@ These functions are built into the program, other [[type:function]]s can be defi
 | [[fun:english_plural|english_singular]]		Find the singular of a word, @"cards" -> "card"@.
 | [[fun:process_english_hints]]				Process the hints left by english_ functions in a keyword's reminder text.
 	
-! Fields and values			<<<
-| [[fun:extra_data]]	Access the value of extra card fields.
+! Fields and values			<<<
 | [[fun:combined_editor|forward_editor]]	Use one field to edit another.
 | [[fun:combined_editor]]		Use one field to edit multiple others.
 | [[fun:primary_choice]]		Return the top level choice chosen from a choice field.