[ticket/11040] search subject and text together

Instead of searching for post_subject and post_text seperately and then
OR them in where clause we use concatenation of the two columns and search

PHPBB3-11040
[ticket/11040] add post_content index

In addition to post_subject and post_title indexes add a post_content
which is concatenation of both the columns.

PHPBB3-11040
[ticket/11445] Remove postgres extra indexes

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

PHPBB3-11445