PR13057 related, remove process.**.return.maxactive(N) probe type

PR13057 related, remove process.**.return.maxactive(N) probe type

The .maxactive stuff is strictly for kretprobes, so should not be
present in process (uprobes) probes.  Use the existing
bind_unprivileged_p flag as a proxy for whether we're dealing with
kernel vs. process probe point hierarchy.

* tapsets.cxx (dwarf_derived_probe::register_*_variants): Make
  bind_unprivileged_p an explicit (non-defaulted) argument.
  Update callers.
  (register_function_variants): Skip TOK_MAXACTIVE variant for process
  probes.