vtls: Use '(void) arg' for unused parameters

vtls: Use '(void) arg' for unused parameters

Prefer void for unused parameters, rather than assigning an argument to
itself as a) unintelligent compilers won't optimize it out, b) it can't
be used for const parameters, c) it will cause compilation warnings for
clang with -Wself-assign and d) is inconsistent with other areas of the
curl source code.