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.

Libraries/LibC/stdio.cpp

@@ -1179,15 +1179,13 @@ int vfscanf(FILE* stream, const char* fmt, va_list ap)
     return vsscanf(buffer, fmt, ap);
 }
 
-void flockfile(FILE* filehandle)
+void flockfile([[maybe_unused]] FILE* filehandle)
 {
-    (void)filehandle;
     dbgprintf("FIXME: Implement flockfile()\n");
 }
 
-void funlockfile(FILE* filehandle)
+void funlockfile([[maybe_unused]] FILE* filehandle)
 {
-    (void)filehandle;
     dbgprintf("FIXME: Implement funlockfile()\n");
 }