Remove duplicate copy of shunit2 (#1086)

The tests use `test/shunit2` so the copy under `vendor/` is redundant.

I've added to the CHANGELOG since `vendor/` is added by the buildpack to
`PATH` (something long term we should stop doing), increasing the chance
an app was accidentally relying upon `shunit2` (albeit still unlikely).

Closes @W-8145517@.

Author: edmorley

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Remove `vendor/shunit2` (#1086).
 - Replace `BUILDPACK_VENDOR_URL` and `USE_STAGING_BINARIES` with `BUILDPACK_S3_BASE_URL` (#1085).
 
 ## v181 (2020-09-29)