Revert "Abstract external_plugin_deps() one level"

Per further discussion, it appears that one can uses aliases in bazel
files that makes this sort of redirection redundant.

```
load(":gitiles/external_plugin_deps.bzl", gitiles = "external_plugin_deps")

def external_plugin_deps():
  gitiles()
```

In the core external_plugin_deps.bzl is perfectly sufficient to accomplish what I'm doing here. We should document this better.

This reverts commit 4310dfca63607cea0e0ae8187f178cd280d285f7.

Reason for revert: <INSERT REASONING HERE>

Change-Id: I6e2ff7115315315cf24d634a0da36083309416be
diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index 3f4d479..d18ead2 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -3,9 +3,6 @@
 COMMONMARK_VERSION = '0.9.0'
 
 def external_plugin_deps():
-  gitiles()
-
-def gitiles():
   maven_jar(
     name = 'gitiles_servlet',
     artifact = 'com.google.gitiles:gitiles-servlet:0.2-5',