pf: remove incorrect fragmentation check

We do not need to check PFDESC_IP_REAS while tracking TCP state.
Moreover, this check incorrectly considers no-data packets (e.g. RST) to
be in-window when this flag is not set.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Approved by: so
Security: FreeBSD-SA-23:17.pf