Delete Stale Metrics Files

Test: Run `m --bazle-mode-dev nothing` again  after `m --bazel-mode-dev nothing`.
bp2build_metrics.pb no longer exists, the rest metrics files
have the timestamp of the latest creation time.

Bug: 256617990
Change-Id: I869d1b9c3abcb517744e66b21e5c12d6b5b1e97b