Remove long-forgotten plugin virtuals.

Summary:
EnablePluginLogging, GetPluginCommandHelp and ExecutePluginCommand aren't
implemented or used anywhere, so remove them from the Hexagon and POSIX
Dynamic Loaders as well as the FreeBSD process.

Reviewers: clayborg, labath, emaste

Subscribers: lldb-commits, emaste

Differential Revision: http://reviews.llvm.org/D13581

llvm-svn: 249840