revert: temporarily revert the update for icon sizes

What does this MR do?

Partially reverts this MR. Update of default-icon-size from 18px to 16px is preserved.

Removal of the 10 && 18 options from $gl-icon-sizes was reverted due to the frequent usage in GitLab codebase of these icon sizes. We'll deprecate them iteratively

Contributes to #1232 (closed)

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