[LLDB] Remove incorrect dotest.py invocation

The invocation shown by dotest.py to re-run a single test is misleading:
it ranges from missing arguments (best case scenario) to being totally
wrong (worst case scenario).

In the past I've tried to get it right, but given the dotest
architecture this is harder than it looks. Furthermore, we have pretty
good documentation on the website [1] for most use cases.

This patch removes the rerun invocation.

[1] https://lldb.llvm.org/resources/test.html