Remove Legacy Codes about trim_empty_channels and trim_obs_channels

[tangxifan]

This PR remove the legacy codes from VPR codes base due to the obsoleted options trim_empty_channels and trim_obs_channels.

Description

The options impact only two parts of VPR:

• The setup function SetupVPR() where the options are loaded
• The routing resource graph builder build_rr_graph() where the options are input arguments and force modification on routing channels.

Related Issue

To address issue #1552

Motivation and Context

The dead codes are no longer used in any tests and their functionality have been fully covered by the custom switch block feature.

How Has This Been Tested?

This PR does just clean-up. The removed codes are never called in any tests.

Types of changes

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[kmurray]

Looks good to me, nice to see this code gone.

[litghost]

LGTM

[vaughnbetz]

Looks good; thanks for cleaning up the code by getting rid of these!