efidev: remove EFIIOC_GET_TABLE ioctl

This ioctl would instantly induce a panic, likely since near inception, up
until 0861c7d3e048. Lack of previous interest in fixing it combined with
the problematic interface (exports a pointer, really a physical address)
brings us to the natural conclusion: remove it until a useful consumer
forward.

If it eventually gets resurrected, the interface should definitely not
return in this exact form and likely needs to be reimagined.

The associated KPI, efi_get_table, is left intact for the time being.

Reviewed by: imp, jrtc27
Also discussed with: brooks, jhb
Differential Revision: https://reviews.freebsd.org/D28030