bench: improve context creation in ECDH benchmark

Calling `secp256k1_context_create` with `SECP256K1_FLAGS_TYPE_CONTEXT`
seems to be not strictly API-compliant, as the only allowed
(non-deprecated) value is `SECP256K1_CONTEXT_NONE`, even if the
former happens to map to the latter currently.

Fix this by not dynamically creating a context in the first place and
switch to using the static context, as it is sufficient for this
benchmark and presumably matches what the "no capabilities" comment
intended back then.

src/modules/ecdh/bench_impl.h

@@ -10,7 +10,7 @@
 #include "../../../include/secp256k1_ecdh.h"
 
 typedef struct {
-    secp256k1_context *ctx;
+    const secp256k1_context *ctx;
     secp256k1_pubkey point;
     unsigned char scalar[32];
 } bench_ecdh_data;
@@ -46,12 +46,9 @@ static void run_ecdh_bench(int iters, int argc, char** argv) {
     bench_ecdh_data data;
     int d = argc == 1;
 
-    /* create a context with no capabilities */
-    data.ctx = secp256k1_context_create(SECP256K1_FLAGS_TYPE_CONTEXT);
+    data.ctx = secp256k1_context_static;
 
     if (d || have_flag(argc, argv, "ecdh")) run_benchmark("ecdh", bench_ecdh, bench_ecdh_setup, NULL, &data, 10, iters);
-
-    secp256k1_context_destroy(data.ctx);
 }
 
 #endif /* SECP256K1_MODULE_ECDH_BENCH_H */