ci: unpin m6i/5.10 AMI

Performance of m6i on Linux 5.10 is stable again.

Signed-off-by: Pablo Barbáchano <[email protected]>

Author: pb8o

.buildkite/pipeline_perf.py

@@ -119,9 +119,7 @@
 # }
 # will pin steps running on instances "m6i.metal" with kernel version tagged "linux_6.1"
 # to a new kernel version tagged "linux_6.1-pinned"
-pins = {
-    "linux_5.10-pinned": {"instance": "m6i.metal", "kv": "linux_5.10"},
-}
+pins = {}
 
 
 def apply_pins(steps):