lib/srdb1/schema: removed kamailio-extensions.xml file

- file didn't have any content itself, tried to include an inexistent
extra.xml, which generated an warning for 'make dbschema'

(cherry picked from commit 367a865e7f0c9cd67fda9176add1ca3c0dad2d46)

src/lib/srdb1/schema/kamailio-extensions.xml

@@ -1,21 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE database PUBLIC "-//kamailio.org//DTD DBSchema V1.1//EN"
-  "http://kamailio.org/pub/kamailio/dbschema/dtd/1.1/dbschema.dtd" [
-
-  <!ENTITY % entities SYSTEM "entities.xml">
-  %entities;
-]>
-
-<!-- Inclusion point for custom extensions for Kamailio -->
-
-<database xmlns:xi="http://www.w3.org/2001/XInclude">
-    <name>CUSTOM Extensions</name>
-    <!-- Include extra definitions if found, it is not
-         an error if they are not present, although it
-         would produce a warning -->
-    <xi:include href="extra.xml">
-        <xi:fallback/>
-    </xi:include>
-    <!-- add more extensions here -->
-    <!-- <xi:include href="more-extras.xml"> -->
-</database>