remove coming soon checkbox

[mwvolo]

⚠️

• Still needs migrations, create them after Alter new edition book state (admin only) #1125 and remove unused ally content field from book #1126
• Merge after those two PRs to prevent migrations headaches
• Verify code on os-webview is no longer referencing this field https://github.com/openstax/os-webview/search?q=coming_soon

[codecov]

Codecov Report

Merging #1128 (0ff2127) into master (5b26351) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1128      +/-   ##
==========================================
- Coverage   83.49%   83.48%   -0.01%     
==========================================
  Files         393      394       +1     
  Lines        6531     6529       -2     
==========================================
- Hits         5453     5451       -2     
  Misses       1078     1078