Revert "support UInt & BigInt in TOML"

[KristofferC]

Reverts #47903

This caused a bunch of type instabilities in the TOML parser.

julia> code_warntype(Base.TOML.parse_datetime, Tuple{Base.TOML.Parser})
MethodInstance for Base.TOML.parse_datetime(::Base.TOML.Parser)
  from parse_datetime(l) @ Base.TOML ~/julia/base/toml_parser.jl:960
Arguments
  #self#::Core.Const(Base.TOML.parse_datetime)
  l::Base.TOML.Parser
Locals
  @_3::Int64
  ms::Int64
  s::Int64
  m::Int64
  h::Int64
  v@_8::Union{NTuple{4, Int64}, Base.TOML.ParserError}
  read_space::Bool
  day::Any
  v@_11::Any
  v@_12::Union{Bool, Base.TOML.ParserError}
  month::Any
  v@_14::Any
  v@_15::Union{Bool, Base.TOML.ParserError}
  year::Any
  v@_17::Any
Body::Any
...

[vtjnash]

Can it be fixed? What is the penalty here for type instabilities? There isn't normally anything especially wrong with having that.

[fingolfin]

What is the status of this? This PR was approved in May 2023 but never merged, and now has a conflict. Do we still need the revert?

[KristofferC]

Let's close for now.