build: Unconditionally load Boost in CMake config

Currently, as @krasznaa discovered, the finding of the Boost dependency
in an installed version of Acts (i.e., through `ActsConfig.cmake` uses
the `@ACTS_USE_SYSTEM_BOOST@` template, but this variable no longer
exists.

CMake, in its infinite wisdom, silently ignores this, producing `if()`,
which it _again_ silently accepts and which apparently evaluates to
false, so Boost is never loaded.

This commit fixes the issue by ensuring that Boost is loaded
unconditionally.

cmake/ActsConfig.cmake.in

@@ -52,9 +52,7 @@ list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/Modules)
 # `find_dependency` is a wrapper around `find_package` that automatically
 # handles QUIET and REQUIRED parameters.
 include(CMakeFindDependencyMacro)
-if(@ACTS_USE_SYSTEM_BOOST@)
-  find_dependency(Boost @Boost_VERSION_STRING@ CONFIG EXACT)
-endif()
+find_dependency(Boost @Boost_VERSION_STRING@ CONFIG EXACT)
 if(@ACTS_USE_SYSTEM_EIGEN3@)
   find_dependency(Eigen3 @Eigen3_VERSION@ CONFIG EXACT)
 endif()