tmpfiles: remove line for automatic clean-ups for /var/cache/man/

Management of /var/cache/man should move to the distribution package
owning the directory (for example, man-db). As man pages are a
non-essential part of the system and unnecessary for minimal setups,
there's no point in having systemd ship these lines.

Distribution packages should make sure the appropriate package for their
distribution adopts this line. Ideally, the line is adopted by the
upstream package.

For Fedora I have filed this bug:

https://bugzilla.redhat.com/show_bug.cgi?id=1110274