build: remove cmake/GenerateHelptags.cmake.in

This file wasn't used since e1cc0fe996.  That may have been
accidental, but it's not needed anymore anyway.  Also the "Workaround
for hanging" is no longer relevant.

cmake/GenerateHelptags.cmake.in

@@ -1,39 +0,0 @@
-if(DEFINED ENV{DESTDIR})
-  file(TO_CMAKE_PATH
-    $ENV{DESTDIR}/@CMAKE_INSTALL_FULL_DATADIR@/nvim/runtime/doc
-    HELPTAGS_WORKING_DIRECTORY)
-else()
-  file(TO_CMAKE_PATH
-    @CMAKE_INSTALL_FULL_DATADIR@/nvim/runtime/doc
-    HELPTAGS_WORKING_DIRECTORY)
-endif()
-
-message(STATUS "Generating helptags in ${HELPTAGS_WORKING_DIRECTORY}")
-if(EXISTS ${HELPTAGS_WORKING_DIRECTORY}/)
-  message(STATUS "${HELPTAGS_WORKING_DIRECTORY} already exists")
-  # If the doc directory already exists, helptags could fail due to duplicate
-  # tags. Tell the user to remove the directory and try again.
-  set(TROUBLESHOOTING "\nRemove \"${HELPTAGS_WORKING_DIRECTORY}\" and try again.")
-endif()
-
-# Workaround for hanging "yes | nvim -c 'helptags ++t .'"
-# and therefore hanging "yes | make install":
-# Set INPUT_FILE to an empty file, causing execute_process
-# to disregard other standard input (such as "yes |").
-set(EMPTY_FILE ${CMAKE_CURRENT_BINARY_DIR}/.GenerateHelptags)
-file(WRITE ${EMPTY_FILE} "")
-execute_process(
-  COMMAND ${CMAKE_CURRENT_BINARY_DIR}/bin/nvim
-    -u NONE
-    -es
-    -c "helptags ++t ."
-    -c quit
-  WORKING_DIRECTORY ${HELPTAGS_WORKING_DIRECTORY}
-  INPUT_FILE ${EMPTY_FILE}
-  OUTPUT_VARIABLE err
-  ERROR_VARIABLE err
-  RESULT_VARIABLE res)
-
-if(NOT res EQUAL 0)
-  message(FATAL_ERROR "Generating helptags failed: ${err} - ${res}${TROUBLESHOOTING}")
-endif()