[lldb] Remove include/lldb/lldb-private.h

The header from 62e0681a does something with
LLVM_FALLTHROUGH. Now that llvm-project has switched to C++17 and
LLVM_FALLTHROUGH uses have been migrated to [[fallthrough]], the header is
unneeded.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D131422