[NFC][libc++] Removes incorrect sliceExpr friend.

There is no type named sliceExpr, so it's likely a misspelling of
either slice_array or __slice_expr. Neither of which appear to
need the friend declaration.

This may indicate a unimplemented bit of `<valarray`, but ¯\_(ツ)_/¯
Nobody uses it anyway.

(Commit message provided by @EricWF.)

This reverts commit e13c43b2.