By now, we have repeatedly failed to provide a nice
cross-platform implementation of `p_fallocate`. Recent tries to
do that escalated quite fast to a set of different CMake checks,
implementations, fallbacks, etc., which started to look real
awkward to maintain. In fact, `p_fallocate` had only been
introduced in commit 4e3949b7 (tests: test that largefiles can
be read through the tree API, 2019-01-30) to support a test with
large files, but given the maintenance costs it just seems not to
be worht it.

As we have removed the sole user of `p_fallocate` in the previous
commit, let's drop it altogether.