Fix "undefined reference to `fmt::v7::detail::basic_data<void>::digits'"

Remove unused FMT_EXTERN_TEMPLATE_API

test/static-export-test/main.cc

@@ -0,0 +1,6 @@
+#include <iostream>
+#include <string>
+
+extern std::string foo();
+
+int main() { std::cout << foo() << std::endl; }