Remove unused variables

[brylie]

While scanning the project with Flake8 (#33), we received several errors related to unused imports.

Goal

Resolve all Flake8 errors related to unused imports.

Task

The following Flake8 errors should be resolved:

• ./api/read.py:156:9: F841 local variable 'user_categories' is assigned to but never used
• ./api/read.py:187:9: F841 local variable 'problems' is assigned to but never used
• ./api/write.py:17:1: F811 redefinition of unused 'require_post_params' from line 5
• ./api/write.py:18:1: F811 redefinition of unused 'JsonResponse' from line 4
• ./api/write.py:19:1: F811 redefinition of unused 'login_required' from line 6
• ./api/write.py:465:29: F841 local variable 'e' is assigned to but never used
• ./api/models/civi.py:134:9: F841 local variable 'owner_id' is assigned to but never used

[tractaylor]

After making the initial changes listed in the Issue, I reran flake and found 2 more F811. Bot are in read.py

./api/read.py:154:9: F841 local variable 'a' is assigned to but never used
./api/read.py:155:9: F841 local variable 'all_categories' is assigned to but never used

I will fix these in this branch as well