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.

Kernel/FileSystem/ProcFS.cpp

@@ -1647,9 +1647,8 @@ KResult ProcFSInode::add_child(Inode&, const StringView&, mode_t)
     return KResult(-EPERM);
 }
 
-KResult ProcFSInode::remove_child(const StringView& name)
+KResult ProcFSInode::remove_child([[maybe_unused]] const StringView& name)
 {
-    (void)name;
     return KResult(-EPERM);
 }