minor symfony#6909 DumpFile() third argument is deprecated and doesn't exists anymore (mickaelandrieu)

This PR was merged into the 2.7 branch.

Discussion
----------

DumpFile() third argument is deprecated and doesn't exists anymore

see symfony#6906

nl;dr; ``dumpFile()`` third argument is deprecated and removed in 3.x, we shouldn't document it.

Commits
-------

155e48b 	DumpFile() third argument is deprecated and doesn't exists anymore in 3.x

Author: wouterj

components/filesystem.rst

@@ -252,8 +252,6 @@ complete new file (but never a partially-written file)::
 
 The ``file.txt`` file contains ``Hello World`` now.
 
-A desired file mode can be passed as the third argument.
-
 Error Handling
 --------------