Revert "ipfix: Export user specified virtual observation ID".

Revert "ipfix: Export user specified virtual observation ID".

This reverts commit 337bebe91c94d9d201e28811c469869d32e978ff, which caused a
crash in test 1048 "ofproto-dpif - Flow IPFIX sanity check" (now test 1051)
with the following backtrace:

 #0 hmap_first_with_hash (hmap=<optimized out>, hmap=<optimized out>,
    hash=<optimized out>) at ../lib/hmap.h:328
 #1 smap_find__ (smap=0x94, key=key@entry=0x817f7ab "virtual_obs_id",
    key_len=14, hash=2537071222) at ../lib/smap.c:366
 #2 0x0812b9d7 in smap_get_node (smap=0x9738a276,
    key=0x817f7ab "virtual_obs_id") at ../lib/smap.c:198
 #3 0x0812ba30 in smap_get (smap=0x94, key=0x817f7ab "virtual_obs_id")
    at ../lib/smap.c:189
 #4 0x08055a60 in bridge_configure_ipfix (br=<optimized out>)
    at ../vswitchd/bridge.c:1237
 #5 bridge_reconfigure (ovs_cfg=0x94) at ../vswitchd/bridge.c:666
 #6 0x080568d3 in bridge_run () at ../vswitchd/bridge.c:2972
 #7 0x0804c9dd in main (argc=10, argv=0xffd8b934)
    at ../vswitchd/ovs-vswitchd.c:112

Signed-off-by: Ben Pfaff <blp@ovn.org>