rcu: Remove references to rcu_migrate_callbacks() from diagrams

This function is gone since:

	53b46303da (rcu: Remove rsp parameter from rcu_boot_init_percpu_data() and friends)

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Documentation/RCU/Design/Memory-Ordering/TreeRCU-callback-registry.svg

@@ -564,15 +564,6 @@
        font-size="192"
        id="text202-7-9-6"
        style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_migrate_callbacks()</text>
-    <text
-       xml:space="preserve"
-       x="8335.4873"
-       y="5357.1006"
-       font-style="normal"
-       font-weight="bold"
-       font-size="192"
-       id="text202-7-9-6-0"
-       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_migrate_callbacks()</text>
     <text
        xml:space="preserve"
        x="8768.4678"

Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg

@@ -1446,15 +1446,6 @@
        font-size="192"
        id="text202-7-9-6"
        style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_migrate_callbacks()</text>
-    <text
-       xml:space="preserve"
-       x="8335.4873"
-       y="5357.1006"
-       font-style="normal"
-       font-weight="bold"
-       font-size="192"
-       id="text202-7-9-6-0"
-       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_migrate_callbacks()</text>
     <text
        xml:space="preserve"
        x="8768.4678"