Remove INSTALLED_VBMETAIMAGE_TARGET dependency from target-files.zip

Remove the INSTALLED_VBMETAIMAGE_TARGET, which depends on all the
other images, it is no longer necessary after
If1eff317621882994e7dcbf37cd35f5bfa16b69a.  This will allow the
next change to parallelize target-files.zip with the image generation
rules.

Test: treehugger
Change-Id: Ie27bdbd2d414e9b475823639a1b3944fe0756d2a