sim: Remove P224 curve references

MCUBoot dropped P224 curve support as
there is little interest in using it.
The simulator threw an error as P224
was never supported, this was removed.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I96011e9b00dbc3a6f7590db365a5f480d85394e1