[execpol.type] Remove "see below" definition from "is_execution_policy" itemdecl (#7909)

We already say that it is a Cpp17UnaryTypeTrait with a base characteristic of either true_type or false_type.

Author: frederick-vs-ja

source/algorithms.tex

@@ -640,7 +640,7 @@
 
 \indexlibraryglobal{is_execution_policy}%
 \begin{itemdecl}
-template<class T> struct is_execution_policy { @\seebelow@ };
+template<class T> struct is_execution_policy;
 \end{itemdecl}
 
 \begin{itemdescr}