build: remove unnecessary unit test code (#21940)

The files generated by test/includes/CMakeLists.txt have been unused
since commit c49e22d396. The unittests include the headers in
test/includes directly without any processing, meaning that the
CMakeLists.txt file is no longer necessary.

test/cmakeconfig/paths.lua.in

@@ -6,7 +6,6 @@ for p in ("${TEST_INCLUDE_DIRS}" .. ";"):gmatch("[^;]+") do
 end
 
 module.test_build_dir = "${CMAKE_BINARY_DIR}"
-module.test_include_path = module.test_build_dir .. "/test/includes/post"
 module.test_libnvim_path = "${TEST_LIBNVIM_PATH}"
 module.test_source_path = "${CMAKE_SOURCE_DIR}"
 module.test_lua_prg = "${LUA_PRG}"

test/includes/CMakeLists.txt

@@ -1,31 +0,0 @@
-file(GLOB_RECURSE PRE_HEADERS RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} pre/*.h)
-
-# We need to add the SDK directories on OS X, and perhaps other operating
-# systems.
-set(gen_cflags)
-foreach(gen_include ${CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES})
-  list(APPEND gen_cflags ${CMAKE_INCLUDE_FLAG_C}${gen_include})
-endforeach()
-
-get_directory_property(gen_cdefs COMPILE_DEFINITIONS)
-foreach(gen_cdef ${gen_cdefs})
-  if(NOT ${gen_cdef} MATCHES "INCLUDE_GENERATED_DECLARATIONS")
-    list(APPEND gen_cflags "-D${gen_cdef}")
-  endif()
-endforeach()
-
-foreach(hfile ${PRE_HEADERS})
-  string(REGEX REPLACE ^pre/ post/ post_hfile ${hfile})
-  get_filename_component(hdir ${CMAKE_CURRENT_BINARY_DIR}/${post_hfile} DIRECTORY)
-  file(MAKE_DIRECTORY ${hdir})
-  add_custom_command(
-    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${post_hfile}
-    COMMAND ${CMAKE_C_COMPILER} -std=c99 -E -P
-      ${CMAKE_CURRENT_SOURCE_DIR}/${hfile}
-      ${gen_cflags}
-      -o ${CMAKE_CURRENT_BINARY_DIR}/${post_hfile})
-  list(APPEND POST_HEADERS ${post_hfile})
-endforeach()
-
-add_custom_target(unittest-headers DEPENDS ${POST_HEADERS})
-set_target_properties(unittest-headers PROPERTIES FOLDER test)