Revert "add unsetindex support to more copyto methods (JuliaLang#51760)"

This reverts commit f0a28e9.

base/abstractarray.jl

@@ -1083,45 +1083,28 @@ function copyto_unaliased!(deststyle::IndexStyle, dest::AbstractArray, srcstyle:
  if srcstyle isa IndexLinear
  # Single-index implementation
  @inbounds for i in srcinds
- if isassigned(src, i)
- dest[i + Δi] = src[i]
- else
- _unsetindex!(dest, i + Δi)
- end
+ dest[i + Δi] = src[i]
  end
  else
  # Dual-index implementation
  i = idf - 1
- @inbounds for a in eachindex(src)
- i += 1
- if isassigned(src, a)
- dest[i] = src[a]
- else
- _unsetindex!(dest, i)
- end
+ @inbounds for a in src
+ dest[i+=1] = a
  end
  end
  else
  iterdest, itersrc = eachindex(dest), eachindex(src)
  if iterdest == itersrc
  # Shared-iterator implementation
- @inbounds for I in iterdest
- if isassigned(src, I)
- dest[I] = src[I]
- else
- _unsetindex!(dest, I)
- end
+ for I in iterdest
+ @inbounds dest[I] = src[I]
  end
  else
  # Dual-iterator implementation
  ret = iterate(iterdest)
- @inbounds for a in itersrc
+ @inbounds for a in src
  idx, state = ret::NTuple{2,Any}
- if isassigned(src, a)
- dest[idx] = src[a]
- else
- _unsetindex!(dest, idx)
- end
+ dest[idx] = a
  ret = iterate(iterdest, state)
  end
  end
@@ -1150,11 +1133,7 @@ function copyto!(dest::AbstractArray, dstart::Integer,
  (checkbounds(Bool, srcinds, sstart) && checkbounds(Bool, srcinds, sstart+n-1)) || throw(BoundsError(src, sstart:sstart+n-1))
  src′ = unalias(dest, src)
  @inbounds for i = 0:n-1
- if isassigned(src′, sstart+i)
- dest[dstart+i] = src′[sstart+i]
- else
- _unsetindex!(dest, dstart+i)
- end
+ dest[dstart+i] = src′[sstart+i]
  end
  return dest
 end
@@ -1165,7 +1144,7 @@ function copy(a::AbstractArray)
 end
 
 function copyto!(B::AbstractVecOrMat{R}, ir_dest::AbstractRange{Int}, jr_dest::AbstractRange{Int},
-  A::AbstractVecOrMat{S}, ir_src::AbstractRange{Int}, jr_src::AbstractRange{Int}) where {R,S}
+ A::AbstractVecOrMat{S}, ir_src::AbstractRange{Int}, jr_src::AbstractRange{Int}) where {R,S}
  if length(ir_dest) != length(ir_src)
  throw(ArgumentError(LazyString("source and destination must have same size (got ",
  length(ir_src)," and ",length(ir_dest),")")))