minor #7865 [Lock] Remove Lock component (jderusse)

This PR was merged into the master branch.

Discussion
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[Lock] Remove Lock component

Remove the documentation of the Lock component as decided in symfony/symfony#22580 and applyied in symfony/symfony#22590.
The documentaiotn will be re-add in 3.4.

Commits
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e14709d Remove Lock component

Author: javiereguiluz