Drop csrf token from API client

[dominik-zeglen]

I want to merge this change because it removes redundant cookies and headers.

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[django-queries]

Here is the report for 0b19a42 (mirumee/saleor @ csrf-token)
Base comparison is 482ba00.

No differences were found. (click me)

# api.benchmark checkout
  test name                           	left count 	right count
  ------------------------------------	-----------	-----------
  add billing address to checkout     	         41	         41
  add shipping to checkout            	          7	          7
  checkout payment charge             	         16	         16
  complete checkout                   	          6	          6
  create checkout                     	         45	         45

# api.benchmark homepage
  test name                           	left count 	right count
  ------------------------------------	-----------	-----------
  retrieve main menu                  	          5	          5
  retrieve product list               	          4	          4
  retrieve secondary menu             	          5	          5
  retrieve shop                       	          2	          2

# api.benchmark product
  test name                           	left count 	right count
  ------------------------------------	-----------	-----------
  product details                     	         13	         13

# api.benchmark variant
  test name                           	left count 	right count
  ------------------------------------	-----------	-----------
  retrieve variant list               	          9	          9

[codecov]

Codecov Report

Merging #4357 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4357      +/-   ##
==========================================
- Coverage   91.97%   91.96%   -0.01%     
==========================================
  Files         285      285              
  Lines       15945    15911      -34     
  Branches     1553     1547       -6     
==========================================
- Hits        14665    14633      -32     
+ Misses        874      873       -1     
+ Partials      406      405       -1

Impacted Files	Coverage Δ
saleor/dashboard/discount/forms.py	97.77% <0%> (-0.29%)	⬇️
saleor/checkout/utils.py	83.2% <0%> (-0.14%)	⬇️
saleor/graphql/discount/mutations.py	98.14% <0%> (-0.04%)	⬇️
saleor/dashboard/order/utils.py	93.42% <0%> (ø)	⬆️
saleor/dashboard/discount/views.py	72.64% <0%> (ø)	⬆️
saleor/graphql/discount/enums.py	100% <0%> (ø)	⬆️

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