FindBoost: Finish reverting "Simplify search in lists" for 3.9

In commit v3.9.2~3^2 (FindBoost: Revert "Simplify search in lists.",
2017-09-05) we forgot to also revert commit commit v3.9.1~15^2
(FindBoost: pop policy stack before returning, 2017-07-24) which had
been made as a fixup to the original commit.

Fixes: #17309

Modules/FindBoost.cmake

@@ -240,8 +240,6 @@ if (NOT Boost_NO_BOOST_CMAKE)
       message("Found Boost components:")
       message("   ${Boost_FIND_COMPONENTS}")
     endif()
-    # Restore project's policies
-    cmake_policy(POP)
     return()
   endif()
 endif()