fastbootd: Remove metadata merging code.

The purpose of this code was to support preserving partitions across a
flashall operation. Since we are not supporting persistent read-write
partitions, this merging code is effectively useless - it is preserving
only partitions that will be deleted.

Additionally, this code does not merge partition group changes, since
doing so accurately is difficult. Partition size changes aren't sent
until after update-super resolves, so a valid re-grouping could be
rejected during update-super if the existing partitions are too big.

This patch removes the update-super merging code until a use case comes
along and we can properly evaluate how it should work.

Bug: N/A
Test: fastboot flashall
Change-Id: I1d622b23dabdf031897be9de49f59fc8cf3caf3b