[build] Clean up redundant LTO configs

We enabled Rust LTO by default in release mode
(https://fxrev.dev/1454713). This CL removes now-redundant
LTO/ThinLTO configs from individual targets to simplify
the build logic.

Bug: 457537341
Change-Id: I5af13fba663b3b1bca790800c6b84004546886f6
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1458328
Reviewed-by: Andrew Davies <awdavies@google.com>
Reviewed-by: Adam Perry <adamperry@google.com>
Reviewed-by: Josh Gargus <jjosh@google.com>
Fuchsia-Auto-Submit: Gulfem Savrun Yeniceri <gulfem@google.com>
Commit-Queue: Gulfem Savrun Yeniceri <gulfem@google.com>
Reviewed-by: Olivia Nordquist <nolivia@google.com>
Reviewed-by: Zeling Feng <zeling@google.com>
Reviewed-by: James Sullivan <jfsulliv@google.com>