scratch: remove quick validity check

scratch: remove quick validity check
Roll back fix for github issue #350

About Scratch Usage:
For compile time, scratch space is strongly recommended to be
allocated immediately after database generation.
For runtime, besides using scratch for corresponding database,
Hyperscan also allows user to use larger scratch space allocated
for another database.
When multiple concurrent threads need to use the same databases
and a new scratch space is required, cloning the largest one is
always safe. This is realized based on API hs_scratch_size() and
hs_clone_scratch().
Behaviors beyond above are discouraged and results are undefined.