build: Remove unneeded ((visibility ("default")))

On non-Windows, we initially had `-fvisibility=hidden` to set gcc's
default to "hidden" together with `((visibility ("default")))` to
override it for our API exports. Then
8854805
got rid of the `-fvisibility=hidden` default by adding
`((visibility ("hidden")))` to all non-static definitions that should
not be exported. But this means we also won't need to set
`((visibility ("default")))` anywhere.

The preprocessor logic could be simplified now but we keep it because
we'll need it in the following commit.

Author: real-or-random

include/secp256k1.h

@@ -155,7 +155,7 @@ typedef int (*secp256k1_nonce_function)(
 /* All cases not captured by the Windows-specific logic. */
 # if defined(__GNUC__) && (__GNUC__ >= 4) && defined(SECP256K1_BUILD)
    /* Building libsecp256k1 using GCC or compatible. */
-#  define SECP256K1_API extern __attribute__ ((visibility ("default")))
+#  define SECP256K1_API extern
 # else
    /* Fall back to standard C's extern. */
 #  define SECP256K1_API extern