imgtool: Remove P224 curve references

Imgtool does not provide support for P224
curve, just a placeholder function that
says it's not currently implemented.
This has now been removed as P224 support
had been removed from the boot code too.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I477d8e273085f38f35eaf9a591584f3e937d748d
diff --git a/scripts/imgtool/main.py b/scripts/imgtool/main.py
index a246bea..8b515d5 100755
--- a/scripts/imgtool/main.py
+++ b/scripts/imgtool/main.py
@@ -1,7 +1,7 @@
 #! /usr/bin/env python3
 #
 # Copyright 2017-2020 Linaro Limited
-# Copyright 2019-2021 Arm Limited
+# Copyright 2019-2023 Arm Limited
 #
 # SPDX-License-Identifier: Apache-2.0
 #
@@ -47,10 +47,6 @@
     keys.ECDSA256P1.generate().export_private(keyfile, passwd=passwd)
 
 
-def gen_ecdsa_p224(keyfile, passwd):
-    print("TODO: p-224 not yet implemented")
-
-
 def gen_ed25519(keyfile, passwd):
     keys.Ed25519.generate().export_private(path=keyfile, passwd=passwd)
 
@@ -65,7 +61,6 @@
     'rsa-2048':   gen_rsa2048,
     'rsa-3072':   gen_rsa3072,
     'ecdsa-p256': gen_ecdsa_p256,
-    'ecdsa-p224': gen_ecdsa_p224,
     'ed25519':    gen_ed25519,
     'x25519':     gen_x25519,
 }