Remove unneeded GAP package `sla` #13342

Job	Run time
doctest (1.9, ubuntu-latest)	25m 42s
doctest (1.10, ubuntu-latest)	25m 28s
doctest (1.11-nightly, ubuntu-latest)	24m 17s
doctest (1.10, ubuntu-latest, depwarn=error)	25m 44s
doctest (1.10, macOS-14)	19m 45s
test (1.6, short, ubuntu-latest)	54m 23s
test (1.6, long, ubuntu-latest)	47m 41s
test (1.9, short, ubuntu-latest)	48m 43s
test (1.9, long, ubuntu-latest)	37m 52s
test (1.10, short, ubuntu-latest)	40m 5s
test (1.10, long, ubuntu-latest)	39m 5s
test (1.11-nightly, short, ubuntu-latest)	47m 5s
test (1.11-nightly, long, ubuntu-latest)	42m 58s
test (1.10, short, ubuntu-latest, depwarn=error)	39m 52s
test (1.10, long, ubuntu-latest, depwarn=error)	38m 55s
test (1.10, short, macOS-14)	37m 6s
test (1.10, long, macOS-latest)	52m 1s
	10h 46m 42s

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