releasetools: Remove the unneeded 'prefix' parameters.

... from the following functions in add_img_to_target_files.py.

  AddSystem
  AddSystemOther
  AddVendor
  AddProduct
  AddDtbo
  AddUserdata
  AddVBMeta
  AddPartitionTable
  AddCache

The last user of the parameter in img_from_target_files.py has been
removed in commit 2bb10970 (in O).

Test: pylint --rcfile=pylintrc add_img_to_target_files.py
Test: Check all the callers to the above functions.
Test: m dist
Change-Id: I551d1683def8f8535062fc90f68dafa0f4252822