Keep PK layer context in the PK layer

Previously we kept the ecdsa context created by the PK layer for ECDSA
operations on ECKEY in the ecdsa_restart_ctx structure, which was wrong, and
caused by the fact that we didn't have a proper handling of restart
sub-contexts in the PK layer.

include/mbedtls/ecdsa.h

@@ -91,9 +91,6 @@ typedef struct
 #if defined(MBEDTLS_ECDSA_DETERMINISTIC)
     mbedtls_ecdsa_restart_det_ctx *det; /*!<  ecdsa_sign_det() sub-context  */
 #endif
-#if defined(MBEDTLS_PK_C)
-    mbedtls_ecdsa_context *ecdsa;       /*!<  used by the PK layer          */
-#endif
 } mbedtls_ecdsa_restart_ctx;
 
 #else /* MBEDTLS_ECP_RESTARTABLE */