don't allow configuring saml_max_message_size
don't allow configuring saml_max_message_size

fixes CORE-1784

we've never adjusted it, and it's causing a thundering herd of connections
on HUP and restart

Change-Id: I2d479be48cb19de6389f03cf25e41154bb12bef6
Reviewed-on: https://gerrit.instructure.com/162333
Reviewed-by: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Tested-by: Jenkins
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms