s4:lib/tls: remove tstream_tls_push_trigger_write step

s4:lib/tls: remove tstream_tls_push_trigger_write step

At the time of https://bugzilla.samba.org/show_bug.cgi?id=7218,
we tested this versions:
    2.4.1 -> broken
    2.4.2 -> broken
    2.6.0 -> broken
    2.8.0 -> broken
    2.8.1 -> broken
    2.8.2 -> OK
    2.8.3 -> OK
    2.8.4 -> OK
    2.8.5 -> OK
    2.8.6 -> OK
    2.10.0 -> broken
    2.10.1 -> broken
    2.10.2 -> OK

These seemed to be the fixes in gnutls upstream.

Change 2.8.1 -> 2.8.2:
http://git.savannah.gnu.org/gitweb/?p=gnutls.git;a=commitdiff;h=28fb34099edaf62e5472cc6e5e2749fed369ea01

Change 2.10.1 -> 2.10.2:
http://git.savannah.gnu.org/gitweb/?p=gnutls.git;a=commitdiff;h=0d07d8432d57805a8354ebd6c1e7829f3ab159cb

This shouldn't be a problem with recent (>= 3.6) versions of gnutls.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15621

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>