libprocinfo: remove error msg for unknown process state.

Unknown process states are expected since libprocinfo doesn't
list all process states in
https://github.com/torvalds/linux/blob/master/fs/proc/array.c#L130.
So remove error message for unknown state.

Bug: none
Test: run libprocinfo_test.
Change-Id: I321893855235367e037afd58ccb63d5140619095