lavfi/field: cleanup unused shorthand and doc.

doc/filters.texi

@@ -3096,7 +3096,8 @@ Extract a single field from an interlaced image using stride
 arithmetic to avoid wasting CPU time. The output frames are marked as
 non-interlaced.
 
-This filter accepts the following named options:
+The filter accepts the following options:
+
 @table @option
 @item type
 Specify whether to extract the top (if the value is @code{0} or

libavfilter/vf_field.c

@@ -107,8 +107,6 @@ static const AVFilterPad field_outputs[] = {
     { NULL }
 };
 
-static const char *const shorthand[] = { "type", NULL };
-
 AVFilter avfilter_vf_field = {
     .name          = "field",
     .description   = NULL_IF_CONFIG_SMALL("Extract a field from the input video."),