Since timer is Supported: but not Required:, you're almost certainly safe to simply remove it in Kamailio, even though, of course, strictly speaking, that is not RFC 3261-compliant proxy behaviour.
As a simple test, try:
request_route { ...
# Handle initial INVITE.
t_on_reply("MAIN_REPLY");
if(!t_relay()) sl_reply_error(); }
onreply_route[MAIN_REPLY] { if(t_check_status("200")) { if_is_present_hf("Supported")) remove_hf("Supported"); } }
This can be fine-tuned later to remove only the 'timer' element specifically.
-- Alex