Revert "Add pre_command for some memory cases to drop cache before test"

[ldoktor]

This reverts commit 7966b4abbd0056050811f8cda6412d03a8d85a34 from https://github.com/autotest/tp-qemu/pull/1889 because drop_caches should never be used for ordinary memory management. It should be only used exceptionally for example to obtain hugepages or for performance testing.

Signed-off-by: Lukáš Doktor [email protected]