[ticket/11621] Remove mysql extra indexes

Remove post_text index as post_content index is sufficient to search post
text.

PHPBB3-11621
Merge remote-tracking branch 'phpbb/develop' into ticket/11621

* phpbb/develop: (542 commits)
  [ticket/11901] Pass empty string as relative_url for non-local magic URL.
  [ticket/11898] Correct adm_relative_path created by installer
  [ticket/11898] Correct admin S_LOGIN_ACTION
  [ticket/11892] Fix undefined variables: to_forum_id & to_topic_id
  [ticket/11890] Fix untranslated string when deleting posts through MCP.
  [ticket/11895] Fix undefined $forum_id
  [ticket/10383] Update the results without the overlay popup.
  [ticket/10383] Make the content transitions smoother.
  [ticket/10383] AJAXify poll voting
  [ticket/11888] Update default search backend in namespaces migration
  [ticket/11739] Rename “Remember Me” login keys module
  [ticket/11893] Remove unnecessary <fieldset> causing a large font size
  [ticket/11894] Fix typo in "Delete post" option
  [ticket/11888] Use \phpbb\search\fulltext_native as search backend config
  [ticket/11608] Mark failed test as incomplete for mysql search backend
  [ticket/11608] Update search backend class names to namespaces
  [ticket/11608] Move markIncomplete to search backends test
  [ticket/11608] Rename base search test class file
  [ticket/11608] Remove suffix test from base class
  [ticket/11608] Mark postgres search not found test as incomplete
  ...
[ticket/11621] Swap columns of post_content index.

PHPBB3-11621
[ticket/11621] Remove unnecessary things around index_created() return value.

PHPBB3-11621
[ticket/11621] Add migration dropping MySQL search indexes.

Possibly time-consuming index-regeneration will be left to the user.

PHPBB3-11621
[ticket/11621] Use effectively_installed() instead.

PHPBB3-11621
Merge remote-tracking branch 'phpbb/develop' into ticket/11621

* phpbb/develop:
  [ticket/11867] Schema files are not created by create_schema_files.php
  [ticket/11905] 3.1.0-a1 Migration
  [ticket/11903] Speed up paging test
  [ticket/11703] Extend the correct migration class.
  [ticket/11874] Correct when $phpbb_root_path is appended to build_url()
  [ticket/11703] Also use empty() for T_JQUERY_LINK.
  [ticket/11703] Add effectively_installed().
  [ticket/11703] Make jQuery CDN switch more generic.
  [ticket/11874] Fix tests
  [ticket/11874] Do not always prepend the web path; only replace phpbb_root_path
  [ticket/11345] Don't include scheme, so HTTPS is used where necessary.
  [ticket/11345] Wrap remote webfonts in S_ALLOW_CDN.
  [ticket/11345] Move css import to template
  [ticket/11345] Add Droid Sans as fallback font
  [ticket/11345] Change font for buttons
[ticket/11621] Put mysql_fulltext_drop into alpha1 migration.

PHPBB3-11621
[ticket/11621] Correct GPLv2 link in file header.

PHPBB3-11621
[ticket/11621] Add explanation to mysql_fulltext_drop.

PHPBB3-11621
Merge pull request #1766 from bantu/ticket/11621

[ticket/11621] Improve MySQL fulltext search indexes