lgc: remove obsolete comment
lgc: remove obsolete comment

This comment is outdated since commit 6675da21156281 ("lgc: set
AMDGPU_xx calling conventions as soon as possible"). Previously, this
pass could be run in one of two different modes, and the comment is a
reference to that.

GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc