chore(measure_theory/function/l1_space): drop `integrable.sub'` (#14309)
chore(measure_theory/function/l1_space): drop `integrable.sub'` (#14309)

It used to have weaker TC assumptions than `integrable.sub` but now it's just a weaker version of it.





leanprover-community/mathlib,leanprover-community/mathlib