README: Remove API documentation, link to stb-tester.com instead

The API documentation on stb-tester.com is generated via Sphinx.

This allows us to get rid of the horrible mess that was `api-doc.sh`.

Makefile

@@ -141,10 +141,6 @@ stbt.1: README.rst VERSION
 	sed -e 's/(callable_,/(`callable_`,/' |\
 	rst2man > $@
 
-# Ensure the docs for python functions are kept in sync with the code
-update-api-docs:
-	STBT_CONFIG_FILE=stbt.conf ./api-doc.sh README.rst
-
 clean:
 	git clean -Xfd || true
 
@@ -361,7 +357,7 @@ install-stbt-camera: $(stbt_camera_files) stbt-camera.d/gst/stbt-gst-plugins.so
 	$(INSTALL) -m 0644 stbt-camera.d/gst/stbt-gst-plugins.so \
 		$(DESTDIR)$(gstpluginsdir)
 
-.PHONY: all clean deb dist doc install install-core uninstall update-api-docs
+.PHONY: all clean deb dist doc install install-core uninstall
 .PHONY: check check-bashcompletion check-hardware check-integrationtests
 .PHONY: check-nosetests check-pylint install-for-test
 .PHONY: copr-publish ppa-publish rpm srpm