Cleaning up the slave directory

[tswilliams]

In discussions & code review related to issue #142 , we spotted a few areas ripe that should be cleaned up:

• ipbus_oob_test.vhd should be deleted (it's an old test slave that's now redundant)
• Move trans_buffer.vhd and trans_buffer_test.vhd under components/ipbus_util, as trans_buffer is a "master" of the transactor rather than a bus slave
• uc_pipe_interface.vhd: Remove the use work.ipbus_trans_decl.all statement, as that package isn't actually used in that file.

[tswilliams]

All of this clean up has now been done in pull request #149