[passwords/android] Drop fallback deletion of downloaded CSV on startup

UMA indicates this code path isn't exercised. Worst case, if the
deletion fails, the CSV can be downloaded again.

Unrelated to that: inline the code that deletes the LoginDatabase
files in browser_prefs.cc and move it to the bottom of the file. This
means the code will go away 1 year from now during browser_prefs
clean-up, without requiring any additional clean-up password manager
code.

OBSOLETE_HISTOGRAM[PasswordManager.AccountLoginData.RemovalStatus]=The deletion is done unconditionally and does not need metrics anymore
OBSOLETE_HISTOGRAM[PasswordManager.ProfileLoginData.RemovalStatus]=The deletion is done unconditionally and does not need metrics anymore
OBSOLETE_HISTOGRAM[PasswordManager.UPM.AutoExportedCsvStartupDeletionSuccess]=The startup CSV deletion was removed

Bug: 442347616
Change-Id: I5c18bdbb9cda6b37ae6f05540746bfb9cc4fe12c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6913683
Reviewed-by: Ivana Žužić <izuzic@google.com>
Reviewed-by: Ioana Pandele <ioanap@chromium.org>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1512375}