tools: remove_and_symlink can be skipped on unix

This is more optimial and this fixes a problem where occasionally
remove_and_symlink will error with:

    gen/compiler_bundle/main.js --sourcemapFile /Users/rld/src/deno --silent
    Traceback (most recent call last):
      File "../../tools/run_node.py", line 18, in <module>
        util.remove_and_symlink(target_rel, "node_modules", True)
      File "/Users/rld/src/deno/tools/util.py", line 103, in remove_and_symlink
        symlink(target, name, target_is_dir)
      File "/Users/rld/src/deno/tools/util.py", line 141, in symlink
        os.symlink(target, name)
    OSError: [Errno 17] File exists

tools/util.py

@@ -92,6 +92,8 @@ def green_ok():
 
 
 def remove_and_symlink(target, name, target_is_dir=False):
+    if os.name != "nt" and os.path.islink(name):
+        return
     try:
         # On Windows, directory symlink can only be removed with rmdir().
         if os.name == "nt" and os.path.isdir(name):