[fir][NFC] Remove explicit num of inlined element for SmallVectors

Remove explicit number of inlined elements for SmallVector.

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D110912