Please try this first:
set dbi_clk_inputs_t [remove_from_collection [all_inputs] [get_ports ahb_clk] ]
Then you can try:
set dbi_clk_inputs [remove_from_collection $dbi_clk_inputs_t $ahb_clk_inputs}]
or
set dbi_clk_inputs [remove_from_collection $dbi_clk_inputs_t [get_ports $ahb_clk_inputs]]