remove promote_rule for strings. fixes #26200. (#26495)

Author: JeffBezanson

base/strings/basic.jl

@@ -194,8 +194,6 @@ Symbol(s::AbstractString) = Symbol(String(s))
 convert(::Type{T}, s::T) where {T<:AbstractString} = s
 convert(::Type{T}, s::AbstractString) where {T<:AbstractString} = T(s)
 
-promote_rule(::Type{<:AbstractString}, ::Type{<:AbstractString}) = String
-
 ## string & character concatenation ##
 
 """

test/strings/basic.jl

@@ -271,9 +271,10 @@ end
 
     @test length(gstr, 1, 2) == 2
 
-    # tests promote_rule
+    # no string promotion
     let svec = [s"12", GenericString("12"), SubString("123", 1, 2)]
-        @test all(x -> x === "12", svec)
+        @test all(x -> x == "12", svec)
+        @test svec isa Vector{AbstractString}
     end
 end