submodules: remove secp256k1-zkp's branch name because it will rotate

see https://github.com/ElementsProject/secp256k1-zkp/issues/68

.gitmodules

@@ -8,7 +8,6 @@
 [submodule "vendor/secp256k1-zkp"]
 	path = vendor/secp256k1-zkp
 	url = https://github.com/ElementsProject/secp256k1-zkp.git
-	branch = secp256k1-zkp
 [submodule "common/defs/ethereum/tokens"]
 	path = common/defs/ethereum/tokens
 	url = https://github.com/ethereum-lists/tokens.git