discovery: remove turn refresh after discarding relay candidate

When a redundant relay candidate is discarded, a turn refresh with zero
lifetime must be sent to release its allocation on the turn server.
Status Job ID Name Coverage
  Build
passed #2618304
build autotools

00:00:57

passed #2618305
build meson

00:00:24

passed #2618307
docker windows 1809
build msvc amd64

00:05:06

passed #2618308
docker windows 1809
build msvc x86

00:05:17

passed #2618306
docker windows 1809
build msys2

00:09:18

 
  Test
passed #2618311
distcheck autotools

00:02:37

passed #2618314
doc-and-install meson

00:00:40

passed #2618309
test autotools

00:01:52

passed #2618310
test autotools valgrind

00:02:30

passed #2618312
test meson

00:00:59

passed #2618313
test valgrind meson

00:01:01