impl: revert addition of generativelanguage

[scotthart]

This library is not yet ready to be released as part of this repo.

fixes #14702

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This change is 

[snippet-bot]

Here is the summary of changes.

You are about to delete 1 region tag.

• google/cloud/generativelanguage/samples/samples.cc:30, tag text_gen_text_only_prompt

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.45%. Comparing base (fded927) to head (cf6478a).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14731   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files        2323     2323           
  Lines      208070   208070           
=======================================
+ Hits       194449   194452    +3     
+ Misses      13621    13618    -3     

Flag	Coverage Δ
	93.45% <ø> (+<0.01%)	⬆️

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[dbolduc]

Remove section header

[scotthart]

Reviewable status: 0 of 89 files reviewed, 1 unresolved discussion (waiting on @dbolduc)

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CHANGELOG.md line 9 at r2 (raw file):

Previously, dbolduc (Darren Bolduc) wrote…

Remove section header

Done.