remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/util/UTF8.cxx

@@ -204,7 +204,7 @@ struct CheckSequenceUTF8 {
 
 template<>
 struct CheckSequenceUTF8<0U> {
-	constexpr bool operator()(gcc_unused const char *p) const noexcept {
+	constexpr bool operator()([[maybe_unused]] const char *p) const noexcept {
 		return true;
 	}
 };