Remove special Hexagon packet traversal code

On Hexagon, breakpoints need to be on the first instruction of a packet.
When the LLVM disassembler for Hexagon returned 32 bit instructions, we
needed code to find the start of the current packet. Now that the LLVM
disassembler for Hexagon returns packets instead of instructions, we always
have the first instruction of the packet. Remove the packet traversal code
because it can cause problems when the next packet has more than one
instruction.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D84966