Remove HDF5.exists deprecation

This was removed in v0.16 of HDF5 and since the present version of JLD specifies this as minimum compatible HDF5 version, it is safe to remove the warning.

src/JLD.jl

@@ -1410,13 +1410,6 @@ include("JLD00.jl")
 ### v0.12.0 deprecations
 ###
 
-import HDF5: exists
-export exists
-@noinline function exists(p::Union{JldFile, JldGroup, JldDataset}, path::String)
-    Base.depwarn("`exists(p, path)` is deprecated, use `haskey(p, path)` instead.", :exists)
-    return haskey(p, path)
-end
-
 @noinline function Base.names(parent::Union{JldFile, JldGroup})
     Base.depwarn("`names(parent)` is deprecated, use `keys(parent)` instead.", :names)
     return keys(parent)