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/tag/Handler.cxx

@@ -41,7 +41,7 @@ NullTagHandler::OnPicture(const char *, ConstBuffer<void>) noexcept
 }
 
 void
-NullTagHandler::OnAudioFormat(gcc_unused AudioFormat af) noexcept
+NullTagHandler::OnAudioFormat([[maybe_unused]] AudioFormat af) noexcept
 {
 }
 

src/tag/Handler.hxx

@@ -124,7 +124,7 @@ public:
 	explicit NullTagHandler(unsigned _want_mask) noexcept
 		:TagHandler(_want_mask) {}
 
-	void OnDuration(gcc_unused SongTime duration) noexcept override {}
+	void OnDuration([[maybe_unused]] SongTime duration) noexcept override {}
 	void OnTag(TagType type, StringView value) noexcept override;
 	void OnPair(StringView key, StringView value) noexcept override;
 	void OnAudioFormat(AudioFormat af) noexcept override;