Stop installing removed "memcache" extra

It was a left-over from a59ac9d823 ,
which removed the useless dependency.

Change-Id: I97f6a4b36b33954a1a427307285eff758e47c088

tox.ini

@@ -11,7 +11,7 @@ setenv =
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
-  .[ldap,memcache]
+  .[ldap]
 commands =
   stestr run {posargs}
 allowlist_externals =
@@ -107,7 +107,7 @@ max-complexity = 24
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
-  .[ldap,memcache]
+  .[ldap]
 commands=
     bash -c "rm -rf doc/build"
     bash -c "rm -rf doc/source/api"