Remove Julia v1.8 version check and precompilation code

Since the current Julia LTS is v1.10, the version check for v1.8 is no
longer needed. Additionally, the precompilation code itself is not
necessary and has been removed entirely.

Changes:
- Removed the @static if VERSION >= v"1.8" check
- Removed the include("precompile.jl") statement
- Deleted the src/precompile.jl file

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

src/RootedTrees.jl

@@ -1613,9 +1613,4 @@ function __init__()
     return nothing
 end
 
-# explicit precompilation on Julia v1.8 and newer
-@static if VERSION >= v"1.8"
-    include("precompile.jl")
-end
-
 end # module