Tools: Do not emit __field_text in JSON parameter documentation

__field_text is basically the raw parsed text, and is used internally
but as far as I can tell it was never meant for external usage, the JSON
emitter already removes other fields that were not intended to be there.
By including the __field_text entry we were doubling the size of the
emitted documentation

Tools/autotest/param_metadata/jsonemit.py

@@ -62,6 +62,10 @@ class JSONEmit(Emit):
                 except KeyError:
                     pass
 
+            # Remove __field_text key
+            if '__field_text' in param.__dict__:
+                param.__dict__.pop('__field_text')
+
             # Get range section if available
             range_json = {}
             if 'Range' in param.__dict__: