tests: Remove bit-rotted support for "lcov".

At one point I got the "lcov" utilities to work well with OVS.  Then I
didn't try to use them again for a year or so, and when I did I found that
it didn't work at all.  I wasn't able to fix the problem easily, so it
seems better to remove the feature than to leave around broken code.

Signed-off-by: Ben Pfaff <blp@nicira.com>