Remove the concept of "separate metadata"

This was an extra level of attribute indirection (features point
to metadata records which point to key and value strings) which was
intended to reduce the size of temporary storage for features with
large numbers of attributes that were also spread across large numbers
of tiles at maxzoom.

For other kinds of features, the extra indirection slowed things down
instead, and, especially when maxzoom guessing was being used, many more
features were having their metadata externalized than could actually
benefit from it.

plugin.cpp

@@ -405,7 +405,6 @@ serial_feature parse_feature(json_pull *jp, int z, unsigned x, unsigned y, std::
 			sf.bbox[0] = sf.bbox[1] = LLONG_MAX;
 			sf.bbox[2] = sf.bbox[3] = LLONG_MIN;
 			sf.extent = 0;
-			sf.metapos = 0;
 			sf.has_id = false;
 
 			std::string layername = "unknown";