Remove adapters from t4_list earlier during detach.

[jhb]

Summary

This ensures the clip task won't race with t4_destroy_clip_table.

While here, make some mutex destroys unconditional since attach
always initializes them.

Test Plan

• kldunload of if_cxgbe panicked due to the clip task racing with the clip table being destroyed
• with this change and the fix for vmem_destroy, kldunload of if_cxgbe no longer panics