[bugpoint] Do not initialize disassembler passes

We added the initilization of disassembler passes in r306208 with the goal to
bring bugpoint in line with 'opt'. However, 'opt' does itself not initialize
dissassembler passes. As our goal was consistency, we drop the initialization
of dissassembler passes again from bugpoint.

Thanks to Chandler for pointing this out!

llvm-svn: 306275