Revert "sysroot: Merge bootconfig-extra from previously staged deployment"

Revert "sysroot: Merge bootconfig-extra from previously staged deployment"

This reverts commit 4c0d3a819033166ec9753ff8c5fdac5db1bac813.

The 3-way merge of bootconfig-extra during staging breaks bootc
loader-entries set-options-for-source when called multiple times on the
same boot.  Each call sets the desired x-options-source-* keys on the
merge deployment's in-memory bootconfig, but the merge logic gives
higher priority to the previously staged data, overriding the caller's
updates.  This corrupts the source-tracking metadata that bootc relies
on to compute override_kernel_argv, causing stale kargs to persist
across reboots.

The scenario this commit was trying to fix (rpm-ostree re-staging after
bootc on the same boot, preserving bootc's extension keys) will need a
different approach that does not conflict with callers that update
extension keys across multiple stagings.

Signed-off-by: Colin Walters <walters@verbum.org>