Run this query after making DB backup.
Use it at your own risk.
Code:
UPDATE post SET pagetext = REPLACE(pagetext,'[HIDE-REPLY]','');
UPDATE post SET pagetext = REPLACE(pagetext,'[HIDE-POSTS]','');
UPDATE post SET pagetext = REPLACE(pagetext,'[HIDE-THANKS]','');
UPDATE post SET pagetext = REPLACE(pagetext,'[HIDE-REPLY-THANKS]','');
UPDATE post SET pagetext = REPLACE(pagetext,'[SHOWTOGROUPS]','');
UPDATE post SET pagetext = REPLACE(pagetext,'[STG]','');
UPDATE post SET pagetext = REPLACE(pagetext,'[HIDE]','');