drop deprecated value if-available for download-ci-llvm option

[onur-ozkan]

It's been 5 months since we deprecated this. It should be fine to drop its support now.

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 35d14dd has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 35d14dd with merge 91d5e4a...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 91d5e4a to master...

[rust-timer]

Finished benchmarking commit (91d5e4a): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[1.6%, 3.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 672.291s -> 674.319s (0.30%)
Artifact size: 315.97 MiB -> 315.93 MiB (-0.01%)