drop fallback block supporting boost pre-1.67.0

io_context appeared in 1.66, and debian 10/ubuntu 20 had 1.67 or newer..

CMakeLists.txt

@@ -581,20 +581,6 @@ find_package (Boost 1.67.0 COMPONENTS date_time filesystem locale program_option
 if (Boost_FOUND)
   include_directories(${Boost_INCLUDE_DIRS})
   set(HAVE_BOOST 1)
-else()
-  find_package (Boost 1.60.0 REQUIRED COMPONENTS date_time regex locale filesystem system program_options)
-  if (Boost_FOUND)
-    include (CheckIncludeFileCXX)
-    set(CMAKE_REQUIRED_FLAGS "-std=c++17")
-    set(CMAKE_REQUIRED_INCLUDES ${Boost_INCLUDE_DIR})
-    check_include_file_cxx("boost/locale.hpp" AUTO_PTR)
-    unset(CMAKE_REQUIRED_FLAGS)
-    unset(CMAKE_REQUIRED_INCLUDES)
-    if(AUTO_PTR)
-      include_directories(${Boost_INCLUDE_DIRS})
-      set(HAVE_BOOST 1)
-    endif()
-  endif()
 endif()
 if (NOT HAVE_BOOST)
 message (SEND_ERROR "A suitable Boost is not installed, and is required. GnuCash requires that Boost be compatible and compiled with C++17. Boost 1.67 is the first compatible release but some distributions have patched earlier ones to work with C++17. Please install it and ensure that the following libraries are built: date_time, filesystem, locale, regex, program_options and system.")