chore: delete orphan ReluBall package

[chipcrowd]

Summary

• Delete unused examples/balls/relu/ (no Core/Chip BallDomain mapping; registry never selects it).
• Remove dead populateReluBallLowerBuckyballToBankSSAPatterns declaration from compiler/.../LowerBuckyball.h.

Nightly audit trial cleanup area 1 (research-then-spread-out). Pure subtraction.

Test plan

• Confirm no remaining ReluBall / balls/relu references outside this PR
• Spot-check pebble linalg_relu / int2fp_relu workloads still present (unrelated)
• CI check workflow on this PR

[shirohasuki]

Toy maintains all balls not maintained by other chips to ensure these balls' functionality. So ReLU ball is still used by Toy.

[chipcrowd]

ack — missed the Toy ownership convention.

Toy is the catch-all for leftover balls (not just the three currently listed in balldomains/default.toml), and chip workloads also pull examples/balls/*/workloads via buckyball_add_ball_workload_subdirs, so ReluBall is in scope for Toy. Deleting it was wrong.

Closing this PR; will keep ReluBall and treat leftover-ball packages as live unless Toy drops them.