Do not install code to build release notes

... because we require only doc requirements.

Also remove explicit usage of requirements.txt because it is now
automatically detected and installed.

Change-Id: Icd6b38ed72f1e845e1071779a0a30264edf345e3
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -7,7 +7,6 @@ constraints =
   {env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 deps =
   -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt
 setenv =
   EVENTLET_TESTS=1
 # Functional tests with Eventlet involve monkeypatching, so force them to be
@@ -48,6 +47,7 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+skip_install = true
 allowlist_externals = rm
 deps = {[testenv:docs]deps}
 commands =