Revert "Assert that mbufs are writable if we write to them"
This reverts commit f08247fd888e6f7db0ecf2aaa39377144ac40b4c.
This assertion is triggered by
ktls_test:ktls_transmit_aes128_cbc_1_0_sha1_control. Remove the assertion until
we fully understand why.
Sponsored by: Rubicon Communications, LLC ("Netgate")