Update plugin-manager plugin to latest master revision

Includes the following commits:

f1d43fe - Remove plugin-manager from the list of core plugins
f0f0a5d - Remove BUCK build
01452a9 - Remove check with hardcoded GERRIT_NEXT_VERSION
825921e - Fix core plugin name extractions without MANIFEST.MF
1c6a1ce - Backport tests from master to stable-2.16

Change-Id: Ida253f9ebb13852f6efcc4da34afdca870e822eb
diff --git a/plugins/plugin-manager b/plugins/plugin-manager
index 2933add..93d8e12 160000
--- a/plugins/plugin-manager
+++ b/plugins/plugin-manager
@@ -1 +1 @@
-Subproject commit 2933add62ecf2cbfc28cfe2cff81ff0e0eecc913
+Subproject commit 93d8e1248478c6b2db0a8c1080f2766406dd213d