[debuginfo-tests] Stop using installed LLDB and remove redundancy

The removed code just replicated what use_llvm_tool does, plus looked
for an installed LLDB on the PATH to use. In a monorepo world, it seems
likely that if people want to run the tests that require LLDB, they
should enable and build LLDB itself. If users really want to use the
installed LLDB executable, they can specify the path to the executable
as an environment variable "LLDB".

See the discussion in https://reviews.llvm.org/D95339#2638619 for
more details.

Reviewed by: jmorse, aprantl

Differential Revision: https://reviews.llvm.org/D102680