[lldb] Remove cmake check for pexpect with LLDB_TEST_USE_VENDOR_PACKAGES (#86962)

The commit 8bed754c was intended to
support the use case where users want to run all the LLDB tests in an
environment where pexpect is not installed. Those users can build with
`-DLLDB_TEST_USER_ARGS=--skip-category=pexpect` to skip pexpect tests,
*but* because we still fail in cmake configuration, they must use
`-DLLDB_TEST_USE_VENDOR_PACKAGES=ON` to avoid failing due to pexpect not
being available.

I would like to remove `LLDB_TEST_USE_VENDOR_PACKAGES` now, but first
I'd like to make sure users w/o pexpect can pass CI with
`-DLLDB_TEST_USE_VENDOR_PACKAGES=OFF
-DLLDB_TEST_USER_ARGS=--skip-category=pexpect`. Once that is done, I am
not aware of any other issues caused by the previous commits, so the
third party tree should be safe to remove.