revert use of mmap-go (#706)
We have a suspicion that when we switched to using the mmap-go library
it contributed to an issue where zoekt-webserver stops responding on
some linux versions. Our suspicion has something to do from us
hardcoding the page size to 4k to asking the kernel and how that
interacts with THP. Given we don't actually build for windows anymore,
we are partially reverting the mmap changes from
7424ac84bab3ec9ae247339909ffd84e5e3b1338
Test Plan: go test ./... on darwin. Then CI for linux testing.