Remove unwind attributes that are no longer needed with CBMC v6 #7646

Job	Run time
benchcomp-tests	3m 43s
documentation	2m 6s
perf	28m 19s
regression (macos-13)	40m 2s
write-json-symtab-regression	17m 38s
regression (ubuntu-20.04)	27m 9s
regression (ubuntu-22.04)	29m 18s
regression (macos-14)	24m 1s
	2h 52m 16s

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