Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Userland/functrace.cpp

@@ -84,7 +84,7 @@ static void print_syscall(PtraceRegisters& regs, size_t depth)
 
 static NonnullOwnPtr<HashMap<void*, X86::Instruction>> instrument_code()
 {
-    (void)demangle("foo"); // Required for linked with __cxa_demangle
+    [[maybe_unused]] auto r = demangle("foo"); // Required for linked with __cxa_demangle
     auto instrumented = make<HashMap<void*, X86::Instruction>>();
     g_debug_session->elf().image().for_each_section_of_type(SHT_PROGBITS, [&](const ELF::Image::Section& section) {
         if (section.name() != ".text")