Fix assertion failure in ConstructFragmentedRangeTombstones() (#12796)

Fix assertion failure in ConstructFragmentedRangeTombstones() (#12796)

Summary:
the assertion `assert(!IsFragmentedRangeTombstonesConstructed(false));` assumes ConstructFragmentedRangeTombstones() is called only once for a memtable. This is not true since SwitchMemtable() can be called multiple times on the same live memtable, if a previous attempt fails. So remove the assertion in this PR and simplify relevant code.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12796

Test Plan: the exact condition to trigger manifest write in SwitchMemtable() is complicated. Will monitor crash test to see if there's no more failure.

Reviewed By: hx235

Differential Revision: D58913310

Pulled By: cbi42

fbshipit-source-id: 458bb9eebcf6743e9001186fcb757e4b50e8a5d2