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;
 	}