docs: Update usage instructions

[alexandear]

This PR removes the usage note explaining how to import this library when Go modules are disabled.

Go modules were introduced in Go 1.11 (released in 2018) and are now the de facto standard. Developers who use this library with Go modules disabled are typically experts and don't need guidance on how to import go-github into their projects.

[gmlewis]

Thank you, @alexandear!
LGTM.
Merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.60%. Comparing base (4d2c2a5) to head (341a8c5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4008   +/-   ##
=======================================
  Coverage   93.60%   93.60%           
=======================================
  Files         207      207           
  Lines       17834    17834           
=======================================
  Hits        16693    16693           
  Misses        939      939           
  Partials      202      202           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.