[lldb][test] Remove LLDB_TEST_USE_VENDOR_PACKAGES (#89260)

The `LLDB_TEST_USE_VENDOR_PACKAGES` has defaulted to `Off` for a while.
Either installing `pexpect` or skipping those tests with
`-DLLDB_TEST_USER_ARGS=--skip-category=pexpect` seems to be enough that
we can fully remove this option.

This patch removes the `LLDB_TEST_USE_VENDOR_PACKAGES` cmake
configuration as well as the associated code to add
`third_party/Python/module` to the python path. I'll do the actual
deletion of `third_party/Python/module` in a followup PR in the
(unlikely, I hope) event this commit needs to be reverted.