arm64: remove exception instruction length assertion

From the (now removed) comment:

• It is unclear in some cases if the bit is implementation defined.
• The Foundation Model and QEMU disagree on if the IL bit should
• be set when we are in a data fault from the same EL and the ISV
• bit (bit 24) is also set.

Instead of adding even more special cases just remove the assertion.

Approved by: andrew
Sponsored by: The FreeBSD Foundation