[lldb] Fix the man page build

In D94489 we changed the way we build the docs and now have some additional
dependencies to generate the Python API docs. As the same sphinx project is
generating the man pages for LLDB it should have in theory the same setup code
that sets up the mocked LLDB module.

However, as we don't have that setup code the man page generation just fails as
there is no mocked LLDB module and the Python API generation errors out.

The man page anyway doesn't cover the Python API so I don't think there is any
point of going through the whole process (and requiring the sphinx plugins) just
to generate the (eventually unused) Python docs.

This patch just skips the relevant Python API generation when we are building
the man page.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D98441