lavfi/field: switch to AVOption-based system

Also remove mention to syntax explanation which is or will be redundant
and possibly confusing.

doc/filters.texi

@@ -3112,17 +3112,6 @@ Specify whether to extract the top (if the value is @code{0} or
 @code{bottom}).
 @end table
 
-If the option key is not specified, the first value sets the @var{type}
-option. For example:
-@example
-field=bottom
-@end example
-
-is equivalent to:
-@example
-field=type=bottom
-@end example
-
 @section fieldorder
 
 Transform the field order of the input video.

libavfilter/avfilter.c

@@ -670,6 +670,7 @@ int avfilter_init_filter(AVFilterContext *filter, const char *args, void *opaque
         !strcmp(filter->filter->name,  "ebur128"   ) ||
         !strcmp(filter->filter->name,  "edgedetect") ||
         !strcmp(filter->filter->name,  "fade"      ) ||
+        !strcmp(filter->filter->name,  "field"     ) ||
         !strcmp(filter->filter->name,  "fieldorder") ||
         !strcmp(filter->filter->name,  "fps"       ) ||
         !strcmp(filter->filter->name,  "frei0r"    ) ||

libavfilter/vf_field.c

@@ -117,5 +117,4 @@ AVFilter avfilter_vf_field = {
     .inputs        = field_inputs,
     .outputs       = field_outputs,
     .priv_class    = &field_class,
-    .shorthand     = shorthand,
 };