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: Ib5ed1a6f951ca3b35aec83bc8953a3a44c37379f
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -9,7 +9,6 @@ constraints =
   {env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 deps =
   -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt
 commands =
   stestr run {posargs}
 
@@ -54,6 +53,7 @@ commands =
   doc8 doc/source
 
 [testenv:releasenotes]
+skip_install = true
 deps =
   -r{toxinidir}/doc/requirements.txt
 commands =
@@ -95,6 +95,7 @@ import_exceptions =
 ignore-path = doc/*/target,doc/*/build*
 
 [testenv:bindep]
+skip_install = true
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed