Rename directory.mojom to match file_util.h naming

To be consistent with the base functions that are being called,
rename functions to be DeleteFile/DeletePathRecursively.
RemoveFile is no longer needed as DeleteFile handles both
deleting files and non-recursive directories.

This is a follow-up to:
https://chromium-review.googlesource.com/c/chromium/src/+/2365358

Bug: 1119547
Change-Id: I4b34722e18d04fec9456753ddde525fa6c672d6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391657
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804783}