minor #19171 Remove "--unpack" option from setup.rst (smnandre)

This PR was submitted for the 6.4 branch but it was squashed and merged into the 6.3 branch instead.

Discussion
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Remove "--unpack" option from setup.rst

The "--unpack" option seems to have disapeared ... some times ago

(i could not find the exact PR on the symfony/flex repository)

Commits
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3726dda Remove "--unpack" option from setup.rst

Author: javiereguiluz

setup.rst

@@ -219,8 +219,7 @@ which in turn installs several packages like ``symfony/debug-bundle``,
 You won't see the ``symfony/debug-pack`` dependency in your ``composer.json``,
 as Flex automatically unpacks the pack. This means that it only adds the real
 packages as dependencies (e.g. you will see a new ``symfony/var-dumper`` in
-``require-dev``). While it is not recommended, you can use the ``composer
-require --no-unpack ...`` option to disable unpacking.
+``require-dev``).
 
 .. _security-checker: