As described in docs/install.html:
Once you have succssfully installed phpBB 2.0.3 you MUST ensure you remove install.php, upgrade.php and update_to_202.php files. Leaving these in place is a very serious potential security issue which may lead to deletion or alteration of files, etc. Additionally you MUST remove the contrib directory once you have utilised any files it contains. This directory may include files which though very useful, could compromise your board or account. Beyond these essential deletions you may also wish to delete the db/schemas and docs/ directories if you wish.