Remove README.rst.in (`make doc` keeps README.rst up-to-date)

Ever since 2204c898, `make doc` updates README.rst (in place) so that
the python API documentation is kept in sync with the docstrings in
`stbt.py`.

882f6b76 added additional information to this process; it's no longer
just the python API documentation, it's also the default values of the
`wait_for_match` parameters (which are now not specified in the python
code, but in `stbt.conf`).

However the *process* by which 882f6b76 updates `README.rst` is
different than the existing process for updating the python api docs in
the same file. This current commit uses the same process for updating
both sections of the documentation. The advantage is that it removes the
almost-identical `README.rst.in` added by 882f6b76, thus removing the
following inconsistency:

                         README.rst.in         README.rst

    python api docs      Contained full but    Contains full python
                         obsolete python api   api docs,
                         docs, *not* updated   updated by `make doc`.
                         by `make doc`.

    stbt.conf defaults   Contained             Contains the values,
                         placeholder.          updated by `make doc`.

It was already somewhat non-standard to commit a generated file
(`README.rst`). Having both `README.rst` and `README.rst.in` in the
repository is even more confusing.
Unit tests for api-doc.sh

api-doc.sh is used by `make doc` to keep README.rst up-to-date with the
python API documentation in stbt.py docstrings, and with the
`wait_for_match` parameter default values in stbt.conf.

api-doc.sh doesn't need much test coverage because README.rst is checked
into version control; so each time you run api-doc.sh, your next `git
diff` would alert you if something had gone wrong (and since we all
check our diffs before committing, we'd never commit undesired changes
to README.rst).

However api-doc.sh could go wrong in one way that wouldn't be detected:
Not actually updating README.rst when the API documentation / default
configuration *had* changed. These tests should cover that.

I had to change the implementation of `python_docstrings` (now called
`substitute_python_docstrings`) so that it doesn't do anything to an
input file without the python api section; this is so that I can test
the `templatematch_params` functionality of api-doc.sh.