Revert use of flashcache_kcopy until we find and fix a subtle bug.

Summary: Revert the use of flashcache_kcopy, and fall back to the
tried and tested use of kernel kcopyd until we find and fix a subtle
bug causing copy job leaks. The commit also removed some old #ifdef'ed
out code.

Test Plan:

Reviewers:

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Task ID: #

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