dhclient: fix pcp:pcp cleanup failure

If isc-dhcp44-server is not installed (as is the case the project's CI
servers) we don't create ngctl.shutdown, causing the 'rm' to fail.
That in turn causes the cleanup function to return non-zero, which
causes kyua to consider the test failed.

Use 'rm -f' instead, so we don't error even if the file doesn't exist.

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