[lldb] [Process/FreeBSDRemote] Replace GetRegisterSetCount()

Replace the wrong code in GetRegisterSetCount() with a constant return.
The original code passed register index in place of register set index,
effectively getting always true.  Correcting the code to check for
register set existence is not possible as LLDB supports only eliminating
last register sets.  Just return the full number for now which should
be NFC.

Differential Revision: https://reviews.llvm.org/D93396