[land-bot] Cleanup unnecessary brace expansion

Summary:
The CI can copy this script out of the source tree if
this protection is necessary. Clearly doing it here is wrong.

I also took this opportunity to clean up some code that wasn't doing anything at all. (BUILD_DIR)

Test Plan:
CONDUIT_TOKEN=<token> ./land-patch.sh Dxxxx with some accepted revision.
The revision will not land, but the patch should be applied and smoke tests run as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7423