build: Simplify modules.gni conditional

Removes the redundant `use_generated_modules_build_file` variable in
favor of using `use_autogenerated_modules` directly. This improves
readability and simplifies the build logic.

Change-Id: I76d1bf32b707dbe253c52ff437dc08c3a17fcd07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965339
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1518510}