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/system/EventFD.cxx

@@ -56,6 +56,6 @@ EventFD::Write() noexcept
 	assert(fd.IsDefined());
 
 	static constexpr eventfd_t value = 1;
-	gcc_unused ssize_t nbytes =
+	[[maybe_unused]] ssize_t nbytes =
 		fd.Write(&value, sizeof(value));
 }