supervisor: drop check for invalid state switch

There is no reason why plan could not be denied after it was approved or
approved after deny.
8 jobs for approval-allow in 4 minutes and 55 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #226933
cppcheck

00:00:34

passed #226934
luacheck

00:00:34

passed #226931
system-tests

00:01:40

passed #226932
system-valgrind-tests

00:02:37

passed #226929
unit-tests

00:01:36

passed #226943
unit-valgrind-tests

00:02:05

failed #226936
unit-valgrind-tests

00:00:40

failed #226930
unit-valgrind-tests

00:01:22