Revert "Make /proc/<pid>/ invisible and restricted for other users." (Closes: #8256).

This reverts commit 20a76c4083fa76f3f4e656fb8023247fa15d6a22.

hidepid != 0 in its current simplistic form can't be supported by systemd:

https://bugzilla.redhat.com/show_bug.cgi?id=1130796#c2
http://lists.freedesktop.org/archives/systemd-devel/2012-October/006860.html