Revert positive flow check

Revert changes for checking whether `MBEDTLS_ECP_RESTARTABLE`
is defined, since it broke the CI. The context is used whether the
restartable feature is defined or not.
diff --git a/library/ecdh.c b/library/ecdh.c
index 5fb06cc..0fed2d1 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -191,7 +191,7 @@
 {
     int ret;
     size_t grp_len, pt_len;
-#if defined(MBEDTLS_ECP_RESTARTABLE)
+#if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;
 #endif
 
@@ -286,7 +286,7 @@
                       void *p_rng )
 {
     int ret;
-#if defined(MBEDTLS_ECP_RESTARTABLE)
+#if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;
 #endif
 
@@ -342,7 +342,7 @@
                       void *p_rng )
 {
     int ret;
-#if defined(MBEDTLS_ECP_RESTARTABLE)
+#if !defined(MBEDTLS_ECDH_COMPUTE_SHARED_ALT)
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;
 #endif