[DOC] fix ignore-persist documentation

During the documentation of the "ignore-persist" keyword, I documented an
invalid "option ignore-persist" and forgot to remove it. It's time to fix it.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index fc1a901..88c4544 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -3343,30 +3343,6 @@
   See also : "option httpclose"
 
 
-option ignore-persist { if | unless } <condition>
-  Declare a condition to ignore persistence
-  May be used in sections:    defaults | frontend | listen | backend
-                                  no   |    yes   |   yes  |   yes
-
-  By default, when cookie persistence is enabled, every requests containing
-  the cookie are unconditionally persistent (assuming the target server is up
-  and running).
-
-  The "ignore-persist" statement allows one to declare various ACL-based
-  conditions which, when met, will cause a request to ignore persistence.
-  This is sometimes useful to load balance requests for static files, which
-  oftenly don't require persistence. This can also be used to fully disable
-  persistence for a specific User-Agent (for example, some web crawler bots).
-
-  Combined with "appsession", it can also help reduce HAProxy memory usage, as
-  the appsession table won't grow if persistence is ignored.
-
-  The persistence is ignored when an "if" condition is met, or unless an
-  "unless" condition is met.
-
-  See also : "option force-persist", "cookie", and section 7 about ACL usage.
-
-
 option independant-streams
 no option independant-streams
   Enable or disable independant timeout processing for both directions