[lldb] Remove the legacy FreeBSD plugin

The new FreeBSDRemote plugin has reached feature parity with the legacy
plugin, so we can finally remove the latter.  The new plugin will
be renamed to FreeBSD in a separate commit to avoid confusion.

Differential Revision: https://reviews.llvm.org/D96555

GitOrigin-RevId: cd443398566b953642ead7c81528ab5b4e211eb9
24 files changed