Revert "core/mount: minimize impact on mount storm."

This reverts commit 89f9752e.

This patch causes various problems during boot, where a "mount storm" occurs
naturally. Current approach is flakey, and it seems very risky to push a
feature like this which impacts boot right before a release. So let's revert
for now, and consider a more robust solution after later.

Fixes #11209.

> https://github.com/systemd/systemd/pull/11196#issuecomment-448523186:
"Reverting 89f9752e and fcfb1f77 fixes this test."