Proto/plugin: Adapt plugins considering validate_operation

The plugins should be adapted to make use of the new Validate_operation.validate_operation, and e.g. stop enforcing 1M since validate_operation does it. Then the following temporary functions in module Validate_operation.TMP_for_plugin should be removed:

• precheck_manager which is used in lib_plugin/mempool.ml (done in !6070 (merged))
• precheck_manager__do_nothing_on_non_manager_op which is used in lib_plugin/RPC.ml (done in !5770 (merged))

This issue originates from the following discussion from !5557 (merged):

• @Saroupille started a discussion:

  Are those functions temporary? If so, I think we should consider opening an issue to track this. I am bit sceptical about the interface because it means the protocol assumes there is a plugin while it is optional and only known from Octez point of view (or Tezedge).

  I have the impression that this code is more like legacy code that needs to be removed at some point, but we don't for the moment for retro-compatibility reasons. For the function precheck_manager in particular, I wonder whether it will be used or not. Because once the prevalidator will use a validation_state, the plugin won't need to check the 1M restriction no?