find_dependency: Remove clearing of unused cmake_fd_exact_arg variable

The variable is unused since commit ab358d6a85 (Improve find_dependency
argument handling, 2017-05-22, v3.9.0-rc1~57^2) where handling of EXACT
keyword in find_dependency was reworked.

Modules/CMakeFindDependencyMacro.cmake

@@ -61,6 +61,5 @@ macro(find_dependency dep)
     endif()
     set(cmake_fd_required_arg)
     set(cmake_fd_quiet_arg)
-    set(cmake_fd_exact_arg)
   endif()
 endmacro()