CI: remove Setup Python action

[Kobzol]

This action was added recently in #125590, but it shouldn't really be needed, as our CI was working fine before without it. Our base Ubuntu 20.04 images use Python 3.8, while this action installed Python 3.12, but we don't really need that.

Since this action does not support ARM yet, this blocks #126113. See #125590 (comment).

r? @jdno

[jdno]

@bors r+ rollup=never

[bors]

📌 Commit 9ce12c4 has been approved by jdno

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9ce12c4 with merge 655600c...

[bors]

☀️ Test successful - checks-actions
Approved by: jdno
Pushing 655600c to master...

[rust-timer]

Finished benchmarking commit (655600c): 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 (primary -1.3%, secondary -3.0%)

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)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	2
Improvements ✅ (secondary)	-3.0%	[-3.0%, -3.0%]	1
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	2

Binary size

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

Bootstrap: missing data
Artifact size: 319.67 MiB -> 319.71 MiB (0.01%)