remove some unnecessary string overloads

[KristofferC]

This removes the following invalidations:

julia> using SnoopCompile;

julia> invalidations = @snoopr using InlineStrings;

inserting string(a::Union{Char, SubString{String}, String}, b::Union{Char, SubString{String}, String}, c::InlineString) @ InlineStrings ~/.julia/packages/InlineStrings/xUsry/src/InlineStrings.jl:620 invalidated:
   backedges: 1: superseding string(xs...) @ Base strings/io.jl:189 with MethodInstance for string(::Char, ::Vararg{Any}) (1 children)
              2: superseding string(xs...) @ Base strings/io.jl:189 with MethodInstance for string(::Any, ::Char, ::Any) (12 children)
              3: superseding string(xs...) @ Base strings/io.jl:189 with MethodInstance for string(::Any, ::String, ::Any) (13 children)
              4: superseding string(xs...) @ Base strings/io.jl:189 with MethodInstance for string(::String, ::String, ::Any) (80 children)
              5: superseding string(xs...) @ Base strings/io.jl:189 with MethodInstance for string(::String, ::String, ::Vararg{Any}) (461 children)
   6 mt_cache

Fixes KristofferC/OhMyREPL.jl#361

[KristofferC]

A bit ironic that the invalidation CI fails when it reports:

Run echo "Invalidations on default branch: 237 (237 via deps)" >> $GITHUB_STEP_SUMMARY
  echo "Invalidations on default branch: [2](https://github.com/JuliaStrings/InlineStrings.jl/actions/runs/9905946948/job/27372841502?pr=78#step:9:2)37 (237 via deps)" >> $GITHUB_STEP_SUMMARY
  echo "This branch: 44 (44 via deps)" >> $GITHUB_STEP_SUMMARY