remove build flavor in favor of build hook

[gilzow]

See #707
Changes build:flavor:composer to build:flavor: none and places composer install into the build hook with the following options:
--no-dev --no-progress --no-ansi --no-interaction --prefer-dist --optimize-autoloader

From #707 , the only thing not included are changes to akeneo and pimcore.

[gilzow]

Moved to draft status until we receive further feedback

[Mupsi]

Hey Paul, I've got bad news: the docs link seems to have changed since you opened this! It's now https://docs.platform.sh/languages/php.html#change-the-build-flavor

[gilzow]

@Mupsi updated.
🤞 they don't change the docs again before we finish this discussion.