build: Remove check_prereq.

check_prereq is a tool for recovery API v2, which has been obselete
since we moved to v3 in [1].

[1] commit e08991e02a7d678f2574e85289a34b2a9a537c82

Bug: 24621915
Change-Id: Ie96348fb56c5a3e029cac2417b59d8eb62dc085b
3 files changed