[llvm] [NFC][BOLT] clean up unused option '--cu-processing-batch-size' (PR #204515)

[Amir Ayupov via llvm-commits]

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@@ -98,8 +98,7 @@
 
 - `--cu-processing-batch-size=<uint>`
 
-  Specifies the size of batches for processing CUs. Higher number has better
-  performance, but more memory usage. Default value is 1.
+  Specifies the size of batches for processing CUs. Now it is not used.
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aaupov wrote:

```suggestion
  Deprecated, has no effect.```

https://github.com/llvm/llvm-project/pull/204515