[lldb] Remove -d(ebug) mode from the lldb driver (#83330)

The -d(ebug) option broke 5 years ago when I migrated the driver to
libOption. Since then, we were never check if the option is set. We were
incorrectly toggling the internal variable (m_debug_mode) based on
OPT_no_use_colors instead.

Given that the functionality doesn't seem particularly useful and nobody
noticed it has been broken for 5 years, I'm just removing the flag.