Remove Julia v1.9 get_extension compatibility code

Since all SciML packages now require Julia v1.10+, we can remove the
compatibility checks for `isdefined(Base, :get_extension)` as package
extensions are available in all supported Julia versions.

This removes unnecessary version checks in extension loading code.

Author: ChrisRackauckas

ext/PlotsExt.jl

@@ -1,6 +1,5 @@
 module PlotsExt
 
-if isdefined(Base, :get_extension)
     using Plots: Plots
 else
     import ..Plots: Plots

src/RootedTrees.jl

@@ -10,10 +10,6 @@ using Latexify: Latexify
 using Preferences: @set_preferences!, @load_preference
 using RecipesBase: RecipesBase
 
-if !isdefined(Base, :get_extension)
-    using Requires: @require
-end
-
 export RootedTree, rootedtree, rootedtree!, RootedTreeIterator,
        ColoredRootedTree, BicoloredRootedTree, BicoloredRootedTreeIterator
 
@@ -1604,16 +1600,7 @@ function __init__()
     Threads.resize_nthreads!(PARTITION_ITERATOR_BUFFER_EDGE_SET_TMP,
                              Vector{Bool}(undef, BUFFER_LENGTH))
 
-    @static if !isdefined(Base, :get_extension)
-        @require Plots="91a5bcdd-55d7-5caf-9e0b-520d859cae80" begin
-            include("../ext/PlotsExt.jl")
-        end
-    end
-
-    return nothing
-end
-
-# explicit precompilation on Julia v1.8 and newer
+    @static # explicit precompilation on Julia v1.8 and newer
 @static if VERSION >= v"1.8"
     include("precompile.jl")
 end