libhwc2.1: Get rid of getPanelFileNodePath

We already have a getPanelSysfsPath method and getPanelFileNodePath
was a thin wrapper over that. Having two methods do essentially the
same thing is confusing, so let's remove the getPanelFileNodePath
and instead introduce an overload of getPanelSysfsPath without
the type parameter.

Bug: 288608645
Test: presubmit
Change-Id: I2a157c669c4ad8950c80f697a74202f42179eb51
4 files changed