tests/unix_seqpacket: remove EMSGSIZE tests

tests/unix_seqpacket: remove EMSGSIZE tests

These tests were not testing conformance to the specification, rather than
the limitation of our implementation.  The specification doesn't say that
a SOCK_SEQPACKET shall ever return EMSGSIZE.  It says:

  The SOCK_SEQPACKET socket type is similar to the SOCK_STREAM type,
  and is also connection-oriented. The only difference between these
  types is that record boundaries are maintained using the
  SOCK_SEQPACKET type. A record can be sent using one or more output
  operations and received using one or more input operations, but a
  single operation never transfers parts of more than one record.
  Record boundaries are visible to the receiver via the MSG_EOR flag
  in the received message flags returned by the recvmsg() function. It
  is protocol-specific whether a maximum record size is imposed.

The EMSGSIZE is specified as 'message is too large to be sent all at once,
as the socket requires'.  Indeed existing implementation that has
unix/seqpacket marked as PR_ATOMIC has such a limitation.  But future
implementation won't have, thus remove the tests.

Reviewed by:		tuexen, asomers
Differential Revision:	https://reviews.freebsd.org/D43756