Warnings will be hard errors from clang-16

[RCasatta]

In the CI there are some warnings:

warning: [email protected]: In file included from depend/secp256k1/src/secp256k1.c:23:
warning: [email protected]: In file included from depend/secp256k1/src/assumptions.h:12:
warning: [email protected]: depend/secp256k1/src/util.h:30:5: warning: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Wimplicit-function-declaration]
warning: [email protected]:     printf("{");
warning: [email protected]:     ^
warning: [email protected]: depend/secp256k1/src/util.h:30:5: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'

If I understand correctly what's happening in a downstream library this will become hard errors from clang-16

Error in downstream library:

warning: In file included from depend/secp256k1/src/secp256k1.c:23:
warning: In file included from depend/secp256k1/src/assumptions.h:12:
warning: depend/secp256k1/src/util.h:30:5: error: call to undeclared library function 'printf' with type 'int (const char *, ...)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
warning:     printf("{");
warning:     ^
warning: depend/secp256k1/src/util.h:30:5: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
warning: 1 error generated.

error: failed to run custom build command for `secp256k1-zkp-sys v0.10.0`

[apoelstra]

Should port rust-bitcoin/rust-secp256k1#735 to here which deletes this (totally unused) code.

[RCasatta]

Fixed by #86