Remove setting demo store option on ecommerce sites by default

[ilyasfoo]

Changes proposed in this Pull Request:

Closes #1515.

PR removes setting woocommerce_demo_store option when ecommerce sites are set up. I assume the demo store notice is no longer relevant (it was implemented 6 years ago).

How to test the changes in this Pull Request:

• Have you followed the Writing high-quality testing instructions guide?

1. Use a WPCOM atomic e-commerce site
2. Open up /_cli in a new tab
3. Run option set woocommerce_demo_store no
4. Run option delete wpcom_ec_plan_theme_defaults
5. Go to frontend homepage of the site
6. Refresh at least once to ensure theme setup is run
7. Observe the notice store notice is not shown at the bottom of the page
8. Run option get wpcom_ec_plan_theme_defaults and observe the value is 1

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.

[github-actions]

Size Change: 0 B

Total Size: 201 kB

ℹ️ View Unchanged

Filename	Size
./build/53.js	1.08 kB
./build/index.css	731 B
./build/index.js	126 kB
./build/marketing.js	58.3 kB
./build/payment-gateway-suggestions.css	1.24 kB
./build/payment-gateway-suggestions.js	6.57 kB
./build/plugins.js	3.93 kB
./build/style-index.css	2.15 kB
./build/style-marketing.css	800 B

_{compressed-size-action}

[moon0326]

LGTM and tested well 👍 🚀