[infra] Remove M108 and M112 milestone configuration.

The last M108 platform CrOS has completed its last refresh on Sept 14,
2023, and M112 has been deprecated from Fuchsia. Turning the milestone projects down.

Change-Id: If25d886eeedd9bc8832fe817d639c6a6efd4b784
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4932604
Commit-Queue: Keybo Qian <keybo@google.com>
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Auto-Submit: Keybo Qian <keybo@google.com>
Cr-Commit-Position: refs/heads/main@{#1209125}

infra/config/generated/luci/commit-queue.cfg

@@ -5120,9 +5120,7 @@ config_groups {
     projects {
       name: "chromium/src"
       ref_regexp: "refs/branch-heads/.*"
-      ref_regexp_exclude: "refs/branch-heads/5359"
       ref_regexp_exclude: "refs/branch-heads/5414"
-      ref_regexp_exclude: "refs/branch-heads/5615"
       ref_regexp_exclude: "refs/branch-heads/5735"
       ref_regexp_exclude: "refs/branch-heads/5845"
       ref_regexp_exclude: "refs/branch-heads/5938"