Remove 2 items from public, to fix 1.12

[mcabbott]

On master, but not on 1.11, Flux cannot be loaded because some symbols are both public and exported:

julia> using Flux
Info Given Flux was explicitly requested, output will be shown live 
ERROR: LoadError: cannot declare Flux.trainables public; it is already declared exported

julia> VERSION
v"1.12.0-DEV.1731"

Tracking things down, this was changed in JuliaLang/julia#53664 . It means that @reexport using Optimisers is fragile -- if Optimisers.jl exports a symbol marked public here, it will immediately break Flux.

(Was not noticed here for ages, Zygote wasn't compiling on master until FluxML/Zygote.jl#1542)

Does not fix CI on master, which really should just not try to load Enzyme at all:

Failed to precompile Enzyme [7da242da-08ed-463a-9acd-ee780be4f1d9] to "/home/runner/.julia/compiled/v1.12/Enzyme/jl_FMXEUb".
ERROR: LoadError: UndefVarError: verbose_stmt_info not defined in Compiler

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.50%. Comparing base (13bb061) to head (9af30ea).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2569   +/-   ##
=======================================
  Coverage   32.50%   32.50%           
=======================================
  Files          34       34           
  Lines        2003     2003           
=======================================
  Hits          651      651           
  Misses       1352     1352           

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