Remove remaining uses of `buttercup--mark-stackframe' #338

Job	Run time
Build and test (24.4)	25s
Build and test (24.5)	25s
Build and test (25.1)	25s
Build and test (25.2)	25s
Build and test (25.3)	26s
Build and test (26.1)	27s
Build and test (26.2)	25s
Build and test (26.3)	31s
Build and test (27.1)	30s
Build and test (28.1)	30s
Build and test (28.2)	32s
Build and test (29.1)	26s
Build and test (29.2)	37s
Build and test (29.3)	40s
Build and test (29.4)	38s
Build and test (release-snapshot)	33s
Build and test (snapshot)	25s
	8m 20s

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