chore(vendor): replace secp256k1-zkp with secp256k1 (v0.3.0)

We don't use any ZKP features at the moment, so we might as well switch
to upstream secp256k1 which does stable releases now.

I am keeping the original submodule name and "zkp" in all integrations,
because it seemed kind of pointless to try to rename/refactor everything,
especially when we might want to switch back to secp256k1-zkp in the future.

.gitmodules

@@ -7,7 +7,7 @@
 	ignore = untracked
 [submodule "vendor/secp256k1-zkp"]
 	path = vendor/secp256k1-zkp
-	url = https://github.com/ElementsProject/secp256k1-zkp.git
+	url = https://github.com/bitcoin-core/secp256k1.git
 [submodule "common/defs/ethereum/tokens"]
 	path = common/defs/ethereum/tokens
 	url = https://github.com/ethereum-lists/tokens.git

vendor/secp256k1-zkp

@@ -1 +1 @@
-Subproject commit 7a30cb0c9d99ab195c461a6fb4e654cd4ef19a8d
+Subproject commit bdf39000b9c6a0818e7149ccb500873d079e6e85