llvm_tools: Clean up start_, end_version entries

This removes any dangling end_version or start_version entry
use in the patch_manager code. This forces all PATCHES.json
to use the version_range schema.

There was also a bug where end_version was being incorrectly
set, which messed with nightly builders.

BUG=None
TEST=patch_manager_unittest.py
TEST=cargo test

Change-Id: If8fde535794b95a2cd80291f61a7b4d782b575ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/3503392
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Jordan Abrahams-Whitehead <ajordanr@google.com>
Tested-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
4 files changed