Remove Project#template_tasks which is not used

This method has not been used anywhere.
Before this patch, 'rspec ./spec/ --seed 57074' raise error

https://travis-ci.org/ESTCOSMO/mitsumottaro/jobs/24506998

There has been some problems as follows,
1. Duplicated 'order by project_tasks.position' in SQL was found.
2. On PGSQL, order by project_tasks.position does not work well sometimes.
3. This 'Project#template_tasks' has not been used.

The 1's problem can be fixed by removing the lambda expression of 'order'
from the declaration of :template_tasks, but 2's problem probably comes from
PostgreSQL behavior and it's hard to fix.

So this patch removes this unused declaration.