[ticket/12250] Remove deprecated phpbb_clean_path function

The function phpbb_clean_path() in functions.php is deprecated as is not
used used 3.0/3.1 core

PHPBB3-12250
[ticket/12250] Remove deprecated phpbb_clean_path function

The function phpbb_clean_path() in functions.php is deprecated and is
replaced by filesystem class. Therefore, this function is moved to the
functions_compatibility file.

PHPBB3-12250
Merge remote-tracking branch 'VarunAgw/ticket/12250' into develop

# By Varun Agrawal
# Via Varun Agrawal
* VarunAgw/ticket/12250:
  [ticket/12250] Remove deprecated phpbb_clean_path function
  [ticket/12250] Remove deprecated phpbb_clean_path function