TCP: remove wrong INSIST(csock->recv_cb != NULL)

TCP: remove wrong INSIST(csock->recv_cb != NULL)

This commit removes wrong INSIST() condition as the assumption that if
'csock->recv_cb != NULL' iff 'csock->statichandle != NULL' is wrong.

There is no direct relation between 'csock->statichandle' and
'csock->recv_cb', as 'csock->statichandle' gets set when allocating a
handle regardless of 'csock->recv_cb' not being NULL, as it is
possible to attach to the handle without starting a read operation (at
the very least, it is correct to start writing before reading).

That condition made `cipher-suites` system test fail with crash on
some platforms in FIPS mode (namely, Oracle Linux 9) despite not being
related to FIPS at all.