SliceTricks: use builtin clear

[TyPh00nCdr]

After filtering without allocating, it's recommended to loop over the underlying slice a and setting all unneeded values to nil manually if they're to be gc'ed.

Instead, one might simply use the builtin clear on the sub-slice starting at the end of b.

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[TyPh00nCdr]

@ianlancetaylor Pinging you, if this will be merged soon?