Remove references to BOARD_BUILD_SYSTEM_ROOT_IMAGE

This variable is always false or empty now.
Remove it so we can mark it as deprecated later.

Bug: 241346584
Test: Presubmit
Change-Id: I110a1e1c07a1debd2c1ce75c70261e62eac8ec5c
Merged-In: I110a1e1c07a1debd2c1ce75c70261e62eac8ec5c
(cherry picked from commit f9b02ea4399a25f4dd1936a75fb02e96b5e1d554)
1 file changed