Remove REMOVE_ABC_BUFFERS support

[oharboe]

TL;DR removes REMOVE_ABC_BUFFERS completely from scripting, will go pr-merge green once designs are updated, ready for review.

Single-concern PR: remove all REMOVE_ABC_BUFFERS support from the flow and documentation. Designs are left alone on purpose.

• floorplan.tcl always runs repair_timing_helper, and the remove_buffers branch is gone.
• Removed from variables.yaml, and variables.json and docs/user/FlowVariables.md were regenerated.

After this PR, REMOVE_ABC_BUFFERS only appears in flow/designs/.

Expected CI

pr-head is expected to fail while designs still set REMOVE_ABC_BUFFERS, because their QoR changes. Once the design PRs below have merged, pr-merge goes green without any further change to this PR. So if we agree on the direction, this can get automerge now and it will land by itself when the prerequisites are done.

Prerequisites (design updates):

• Drop REMOVE_ABC_BUFFERS from designs that no longer need it #4575
• Drop REMOVE_ABC_BUFFERS from ihp-sg13g2/aes and riscv32i (needs rules update) #4576
• Drop REMOVE_ABC_BUFFERS from ihp-sg13g2/jpeg (needs judgement) #4577
• Drop REMOVE_ABC_BUFFERS from sky130hd/ibex and sky130hs/ibex (CI to judge) #4578
• Drop REMOVE_ABC_BUFFERS from gf12 ariane, ariane133 and swerv_wrapper (private CI to judge) #4579
• flow/designs/asap7/minimal/config.mk (REMOVE_ABC_BUFFERS ?= 1) is not covered by any of the above yet

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request removes the deprecated REMOVE_ABC_BUFFERS configuration variable from the documentation, variable schemas, and scripts. In floorplan.tcl, setting this variable now raises an error indicating it is no longer supported, and the timing repair helper is executed unconditionally. Feedback on the changes suggests improving the Tcl conditional check for REMOVE_ABC_BUFFERS to robustly handle non-numeric boolean strings (like 'false' or 'no') to avoid triggering false-positive errors.

[openroad-ci]

🔍 QoR check

Metrics reflect the PR merge build — i.e. what will land on the target branch.

Commit eb221d5 · Jenkins build #2 · Baseline: build · View build on dashboard

34 design(s) checked — 1 with regression(s), 0 without a comparable baseline.

❌ ihp-sg13g2/aes base — 1 failing metric(s)

Metric	target	base	delta	limit	band
cts__timing__setup__ws	-0.624158	-0.518657	20.341188878198885%	-0.618657	PeriodPadding 5.0%