[ticket/11237] Fix PHP error in acp_prune.php

Also making the code a bit more efficient (removing one SQL query)

PHPBB3-11237
[ticket/11236] Correct HTML

PHPBB3-11236
[ticket/11236] Do not require group selection in prune users

PHPBB3-11236
[ticket/11236] Change PRUNE_USERS_GROUP_EXPLAIN language

PHPBB3-11236
Merge PR #1370 branch 'EXreaction/ticket/11237' into develop

# By Nathaniel Guse
# Via Nathaniel Guse
* EXreaction/ticket/11237:
  [ticket/11237] Fix PHP error in acp_prune.php
Merge PR #1371 branch 'EXreaction/ticket/11236' into develop

# By Nathaniel Guse
# Via Nathaniel Guse
* EXreaction/ticket/11236:
  [ticket/11236] Change PRUNE_USERS_GROUP_EXPLAIN language
  [ticket/11236] Do not require group selection in prune users
  [ticket/11236] Correct HTML