Vulkan: remove the fallback path to load Vulkan driver

This fallback loading path was for backwards compatibility with devices
being upgraded from pre-Oreo (i.e. pre-Treble) to Oreo or later. Those
devices weren't required to follow a lot of the Treble rules, so linker
namespaces (or binaries compatible with them) couldn't be enforced. This
change removes it so that we can discover issues around loading Vulkan
driver from sphal namespace.

Bug: 156021362
Test: build, flash and boot
Change-Id: I505bdbdb40a06e1d837f1d0b75822b0c60de96c9