Revert inadvertant change in pg_config behavior

In commit a5c43b88 the behavior of command line pg_config was
inadvertantly changed to include the config name when specific
configs are requested, similar to when none are requested and
all are emitted. This breaks scripts that expect to use
pg_config for e.g. PGXS. Revert the behavior to the previous.

src/bin/pg_config/pg_config.c

@@ -121,7 +121,7 @@ show_item(const char *configname,
 	for (i = 0; i < configdata_len; i++)
 	{
 		if (strcmp(configname, configdata[i].name) == 0)
-			printf("%s = %s\n", configdata[i].name, configdata[i].setting);
+			printf("%s\n", configdata[i].setting);
 	}
 }