build: Remove visibility workaround for Windows

This is no longer necessary. While ((visibility("hidden"))) will also
trigger the bug, this attribute is only applied when
SECP256K1_FORCE_HIDDEN_VISIBILITY is set, which is specific to ELF.

This partially reverts commit 447334c.

Author: real-or-random

include/secp256k1.h

@@ -134,15 +134,6 @@ typedef int (*secp256k1_nonce_function)(
      * 1. If using Libtool, it defines DLL_EXPORT automatically.
      * 2. In other cases, SECP256K1_DLL_EXPORT must be defined. */
 #   define SECP256K1_API extern __declspec (dllexport)
-#  else
-    /* Building libsecp256k1 as a static library on Windows.
-     * No declspec is needed, and so we would want the non-Windows-specific
-     * logic below take care of this case. However, this may result in setting
-     * __attribute__ ((visibility("default"))), which is supposed to be a noop
-     * on Windows but may trigger warnings when compiling with -flto due to a
-     * bug in GCC, see
-     * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116478 . */
-#   define SECP256K1_API extern
 #  endif
   /* The user must define SECP256K1_STATIC when consuming libsecp256k1 as a static
    * library on Windows. */