Remove build system for sip v4 (frescobaldi#59)

The last bugfix release of sip 4 was in February 2018, 5 years ago. Even
sip 5 is now unsupported. It should be safe to remove the parallel build
system for sip 4 in favor of the sip 5/6 build system defined in
pyproject.toml and project.py, avoiding the need to maintain two build
systems in parallel.

INSTALL

@@ -4,63 +4,41 @@ Installation:
 
 python-poppler-qt5 installs a single Python module named 'popplerqt5'.
 
-The setup script uses distutils. Use the following commands to build and install
-the module:
-
-    python setup.py build
-    python setup.py install
-
-To install in /usr instead of /usr/local:
-
-    python setup.py install --prefix /usr
-
-
-You can also use the newer way of installing using pip:
+You can install it using pip:
 
     python -m pip install .
 
 
 Troubleshooting:
 ================
 
-The setup script uses pkg-config to determine the version of the installed
+The project.py script uses pkg-config to determine the version of the installed
 poppler-qt5 library. Methods or classes that are not in the installed poppler-
 qt5 library will then also be disabled in the Python binding.
 
 If your system does not provide pkg-config, you can manually specify the poppler
 version to use with the --poppler-version option, e.g.:
 
-python setup.py build_ext --poppler-version=0.14.0
-
-Note that for these options to work you need the 'build_ext' command rather than
-the 'build' command.
+python -m pip install --config-setting "--poppler-version=0.14.0" .
 
 You may then also need the --include-dirs and --library-dirs options to the
 build_ext command if the correct poppler-qt5 header files and libraries can't
 be found.
 
 See the output of
 
-python setup.py --help build_ext
+sip-build --help
 
 for more options.
 
-If you have a Debian-based system such as Ubuntu, and you get the error
-message "ImportError: No module named popplerqt5", try:
-
-python setup.py install --install-layout=deb
-
-See the distutils documentation for more install options.
-
-
 Dependencies:
 =============
 
-- sip 4.9.1 or newer
+- sip 5 or newer
 - poppler-qt5 0.12.0 or newer (poppler-qt5 is part of poppler)
 - Qt5
 - PyQt5
-- Python (tested with 2.6)
+- Python
 
 To build the popplerqt5 module, header files of Qt5 and poppler-qt5 and sip
 files of PyQt5 are needed (sometimes packaged separately in corresponding -dev