Unity: Do not blanket-ignore specific plugins, that's a user choice

Only people building Unity plugins will need to import the asset store tools plugin into their project, and it's a toss up whether they want to commit the tooling into their repo (they might or they might not want to, it highly depends on their workflow). The default .gitignore shouldn't be making this choice for these users.

Unity.gitignore

@@ -4,7 +4,9 @@
 [Bb]uild/
 [Bb]uilds/
 [Ll]ogs/
-[Aa]ssets/AssetStoreTools*
+
+# Uncomment this line if you wish to ignore the asset store tools plugin
+# [Aa]ssets/AssetStoreTools*
 
 # Visual Studio cache directory
 .vs/