Remove MACRO_PLACE_CHANNEL variable

[AcKoucher]

Description

This variable is part of the support for the old macro placer which is being removed in #6047. It's usage over-complicates the halo/blockage halo settings for macro placement and placement blockages generation.

Suggested Solution

Leave only MACRO_HALO and MACRO_BLOCKAGE_HALO.

Additional Context

No response