[cleanup] Delete unused methods of FloatLineGap and FloatLineSpacing

For line_spacing_ and line_gap_, there are two methods for each:
- LineGap and FloatLineGap
- LineSpacing and FloatLineSpacing.

Since callers only ask for their values as integers, this CL deletes
the unused ones.

Bug: n/a
Change-Id: I47b5a578d8f9eb7a4174e1cc323bf59d8e1365b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4751024
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1179567}