Code cleanups

Code cleanups

* Drop unused (*arg) usage from generate_main_syslinux_config()
* Drop global variable DATESTAMP, leftover from commit 100193b24,
  also remove corresponding `import datetime`
* Drop global variable GPT_HEADER, leftover from commit 705a96b84
* Drop unused `import time`
* Drop unused value variable from syslinux_warning() + grub_option()
* Drop unused function update_grml_versions(), leftover from commit 35feaad92af2e
* Drop unused array2string + string2array functions (introduced in commit 68df4a8c90),
  also remove corresponding `import struct`
* Drop unused is_writeable() function
* Drop unused generate_isolinux_splash() function
* Mark unused variables in search_file() as such
  (Vulture will ignore these variables if they start with an underscore)
* Mark unused function and excinfo parameters in del_failed() as unused
  The path and exception information is reported as unused by
  vulture, though shutil.rmtree needs to invoked with three parameters,
  quoting from https://docs.python.org/3/library/shutil.html:

  | If onerror is provided, it must be a callable that accepts three parameters: function, path, and excinfo.
* The 'warn' function is deprecated, using 'warning' instead

Thanks to vulture tool (https://pypi.org/project/vulture/).

Thanks: Chris Hofstaedtler for review + feedback