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/filter/Observer.cxx

@@ -43,7 +43,7 @@ public:
 		observer.proxy = nullptr;
 	}
 
-	void Clear(gcc_unused Proxy *_child) noexcept {
+	void Clear([[maybe_unused]] Proxy *_child) noexcept {
 		assert(child == _child);
 		child = nullptr;
 	}

src/filter/plugins/NormalizeFilterPlugin.cxx

@@ -53,7 +53,7 @@ public:
 };
 
 static std::unique_ptr<PreparedFilter>
-normalize_filter_init(gcc_unused const ConfigBlock &block)
+normalize_filter_init([[maybe_unused]] const ConfigBlock &block)
 {
 	return std::make_unique<PreparedNormalizeFilter>();
 }

src/filter/plugins/NullFilterPlugin.cxx

@@ -38,7 +38,7 @@ public:
 };
 
 static std::unique_ptr<PreparedFilter>
-null_filter_init(gcc_unused const ConfigBlock &block)
+null_filter_init([[maybe_unused]] const ConfigBlock &block)
 {
 	return std::make_unique<PreparedNullFilter>();
 }