Lib_benchmark: Remove manual GC calls

What

Remove manual GC calls from lib_benchmark

Why

Manual GC calls create spurious benchmarks in OCaml 5.2.1 and this needs to be investigated but we still need to be able to see if the benchmarks are slower and we can't currently make this kind of observation with this behaviour. Removing manual GC calls allow us to have readable benchs back.

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR