Don't exit loop early in __wt_tiered_flush_work_wait

Structurally dead code

Actions intended to be performed by the unreachable code will never occur. Code block is unreachable because of the syntactic structure of the code
/src/tiered/tiered_work.c:147: UNREACHABLE 124119 Since the loop increment "entry = entry->q.tqe_next;" is unreachable, the loop body will never execute more than once.